Do Fish Sleep?

Everyone needs sleep. Every night average people perform a sleep ritual: We change into pajamas, crawl into our soft comfy beds, close our eyes and enter into a restful state. Our hearts slow down, we breathe slowly and regularly, and our muscles become relaxed. Once or twice an hour we roll over, but we are no longer tuned in to our environment. We spend about 8 hours a day sleeping – that’s one third of our lives.

Sleep means different things to different forms of animal life. The Saunders Comprehensive Veterinary Dictionary sums it up pretty well: A period of rest during which volition and consciousness are in partial or complete abeyance and the bodily functions partially suspended; a behavioral state marked by characteristic immobile posture and diminished but readily reversible sensitivity to external stimuli.

Most animals have some daily pattern of rest and activity, and in many species these daily cycles are similar to people running around during the day then lying down at night and doing nothing or sleeping. It is believed that fish are no different, although it is a controversial subject. Some fish keep very still, experiencing a quiet period (quiescence) that you might call sleep. Scuba divers often handle reef fish in the middle of the night without startling them and can even lift some species out of the water before they awaken. Tropical freshwater fish in home aquaria appear to be resting immediately after turning the lights on in a room that has been darkened for several hours. Unfortunately, fish have no eyelids so it is difficult to tell whether they are asleep or not.

Why We Sleep

No one knows for sure why creatures sleep. But there are two basic theories:

  • Sleep has a restorative function – It’s possible that sleep helps the body recover from all the work it did while it was awake.
  • Sleep has an adaptive function – It’s possible that they sleep in order to protect themselves or conserve energy. When fish move into their hiding places to rest, their body needs are lessened and they avoid getting eaten.
  • Different Strokes

    Being asleep can mean different things to different fish. Some fish and amphibians reduce their awareness but do not ever become unconscious like the higher vertebrates do. Fish have time periods when they become less aware of their surroundings but their brain waves do not change, and they do not exhibit REM sleep. They aren’t quite asleep but they don’t seem to be fully awake either.

    Some fish undergo a yearly sleep cycle. They hibernate and their metabolic rate slows down. Although they do not hibernate like mammals, as environmental temperatures fall, their metabolic rate and activity decrease, and they go into a stupor and stop feeding. They usually adopt a position towards the bottom of the pond.

    Some fish practice estivation, a state of torpor or dormancy in which they spend time during hot, dry periods to protect themselves from dehydration. The African lungfish buries itself in mud and survives the dry season protected by a cocoon of mud in the riverbed. Carp spend the winter partly buried in lake mud, and in tropical countries many fish sleep, or estivate, through the summer months when swamps and rivers dry up. Walking perch and lungfish bury themselves in mud, leaving only an airhole open, and breathe by means of their lungs. One of the gobies of the Ganges River delta digs a burrow and sleeps through the dry months with only the tip of its tail touching the water. It apparently breathes through its tail.

    Some fish make elaborate preparations for sleep. In David Feldman’s book When Do Fish Sleep?, a scientist describes the nightly ritual of a tired parrotfish that lives in reefs near shore. The parrotfish squeezes into a crevice on the reef. Once settled in, it begins oozing a jelly-like mucus, which forms a protective membrane over his body, and then he nods off into a deep sleep.

    Some fish are motionless in the water during the night, while other fish, like rockfish and grouper, don’t appear to sleep at all. They rest against rocks, bracing themselves with their fins. Some freshwater fish, like catfish, swim up under a log or river bank for shelter during the day.

    Finally, some fish don’t hide the fact that they take an occasional nap. One of the favorite habits of the clown loach, which has alarmed most new clown loach keepers in the past, is that of resting on the bottom of the aquarium on their sides. They appear as though they are dead or sick, but this is just one of the positions that they adopt when resting.

    It’s probable that fish do sleep in some form, whether slowing down or coming to a complete stop, whether hiding or doing it right in the open. But when they sleep the slightest ripple in the water will disturb them. Nevertheless, in some way they rest, just as we do.

    The Great White Shark

    It’s time to give the great white shark a break. Thanks to the movie industry that has depicted great whites as man-eating machines, this magnificent animal may be the most frightening shark to all humans. But although the species is responsible for an average of two to three non-fatal attacks on swimmers, surfers, and divers each year, its role as a menace is greatly exaggerated. More people are killed in the United States each year by dogs than have been killed by sharks in the last hundred years.

    Try looking at the great white with an open mind. They are truly amazing creatures. They have roamed the ocean for at least 400 million years and have earned their title of King of the Ocean. During this time they have changed little.

    The upper teeth are large, broad and triangular, while the lower teeth are slightly more slender. All teeth are serrated. Like other sharks, the great white continually loses its teeth and replaces them with new ones. Their eyelids are fixed in their sockets, but they have excellent eyesight and can hear well enough to detect prey at great distances. Their sense of smell, which improves with hunger, is quite sharp. Even more prominent is their ability to detect other creatures’ electronic fields.

    Great whites are the strongest swimmers of all sharks and cruise at speeds of less than 1 mph. They are not man eaters, but they are meat eaters. Along the west coast, they feed on sea lions and seals. In the western north Atlantic they feed primarily on dead whales. Meals are infrequent and attacks consist of one massive bite taken from under or behind the chosen prey. Humans who have been bitten once generally can escape if they can make it into a boat or back to shore. Sharks usually lose interest after one bite.

    As highly adapted predators, sharks play a key role in maintaining the balance in aquatic environments. However, some specialists believe they are endangered due to shrinking food sources and overfishing by trophy hunters. To remedy this situation, the U.S. Senate recently passed legislation to prohibit shark finning in all U.S. waters, and the state of California has placed the white shark on the protected species list, which means they are now legally protected from unlawful killing or exploitation.

    References for the Saltwater Aquarium Hobbyist

    Many books are written about the care and techniques used in saltwater aquarium systems. Some, of course, are more advanced than others, since an entire branch of aquaculture science involves only the care of saltwater species in captivity.

    The people who practice this science are called aquarists and usually work in exhibit aquariums such as Sea World, the Oceanarium in Lisbon, Portugal, or the Baltimore Aquarium. Their skills are usually a combination of engineering and biology. They learn not only how to create and run the equipment needed for aquaria that are hundreds of thousands-sometimes millions-of gallons in size but also to care for the individuals; they must be aware of specific needs and incompatibilities.

    In larger exhibit aquariums, some aquarists have the task of nursing species back to health, or getting them to breed or spawn. Some species, such as open ocean fish like billfish, have never been kept alive in captivity for more than a couple of days (and being able to do so was significant enough to produce a masters thesis). Such studies are important to the overall health of our oceans, and they provide us with information to preserve these creatures in a clean, healthy protected environment. Many feel that in the coming years, an aquarium may be the only place where we will be able to keep alive species that are becoming extinct in their natural environments today from pollution and over-exploitation.

    Having a saltwater aquarium is, therefore, much more than just keeping some maritime pets. For those of you who have a serious advanced tank set-up and who care for unusual or rare species, it is highly suggested that you get in touch with an aquarist to discuss proper care and potential problems. They understand the difficulty in having a small system (larger volumes of water are easier to control) and they can relate to the dedication you may feel to create a healthy environment for your pets.

    To get in touch with an aquarist, check out some aquarium web sites. Sometimes aquarists are listed, sometimes you may have to go through their PR department to reach one; but most often, these people are happy to assist you. Take most of what you find on the web with a grain of salt, however. Many web sites are more like talk-forums where people share their experiences and may not be the most reliable sources.

    The following is a list of books that were not only helpful in putting together the information on saltwater systems and species that you will find on the database, but also could be a valuable resource for the hobby aquarist. Books you may easily find in stores or on the Internet today are not listed here; instead, the following books were selected as a supplement to what you may already have or are particularly reliable and comprehensive.

    Simon & Schuster’s Guide to Freshwater and Marine Aquarium Fishes, Simon & Schuster, New York, NY, 1976. This is a particularly good guide first written in Italian and translated in 1977. The book lists species of both fresh and marine fishes as well as plants, amphibia, reptiles and invertebrates and gives no more than raw description of the creatures’ needs along with a photo for ID.

    The Saltwater Aquarium Handbook, by George Blasiola, Barron’s Educational Series, Inc., Hauppauge, NY, 2000. This is a very good book for a general overview of marine systems and species, as well as some plants. Especially good for the beginner or someone who may be considering becoming one, but for those needing more specifics, it won’t be enough.

    A Fishkeepers Guide to Marine Fishes, by Dick Mills. Tetra Press, Blacksburg, VA, 1996. This book is a handy reference for the novice to experienced hobbyist regarding the practical aspects of saltwater tanks; however its species list is weak in that very few are described.

    Dynamic Aquaria: Building Living Ecosystems, by Walter Adey and Karen Loveland, Academic Press: San Diego, 1998. An excellent source for both the professional aquarist and for the advanced do-it-yourself salt-water hobbyist, this book is particularly good for the increasingly popular aquarium hobby of designing a tank and creatures around an ecosystem.

    Reef Fish Behavior: Florida/Caribbean/Bahamas by Ned Deloach, New World Publications, Inc.: Jacksonville, FL, 1999. This is a great book on typical reef fish behavior, although more geared for observations in a natural environment.

    The Reef Set (Reef Coral Identification, Reef Fish Identification, Reef Creature Identification: Florida/Caribbean/Bahamas) by Paul Humann. New World Publications, Inc., Jacksonville, FL, 1996. This three-volume set is helpful to those who want to learn about marine life although these books are not at all related to how to keep them. It is great for those hobbyists who need to ID fish in the wild or who want to know more about fish in general such as how they are catagorized. The set is also useful when investigating which fish to buy or to make sure you’re buying what the store maintains its selling.

    Captive Seawater Fishes: Science and Technology, by Stephen Spotte, John Wiley & Sons, Inc.: New York, 1992. This is a text book geared toward professional aquarists or very advanced hobbyists that describes methods and technologies used in aquarist science. It describes methods on how to create specially designed systems to keep saltwater fish, or such things how to raise mass quantities of brine shrimp that will result in a specific nutritional quality, and more.

    Encyclopedia of Fishes by John R. Paxton and William N. Eschmeyer, Academic Press: San Diego, 1998. This is a great reference on the natural history of fishes.

    SeaLife: A Complete Guide to the Marine Environment by Geoffrey Waller (editor), Smithsonian Institution Press: Washington, D.C., 1996. This is an excellent source on the biology and biodiversity of marine life. Not geared toward the aquarium hobby, but a must-have reference for people who are interested in the ocean environment and its biodiversity.

    The Stonefish – The Deadliest Fish in The World

    In an ancient dance ritual performed by the Australian Aborigines, a man wades into the tide pools in search of fish. Suddenly he steps on something – a clay model of a stonefish with 13 wooden dorsal fins – and screams in pain. The dancer writhes on the ground in agony, and the ritual ends sadly with a death song.

    The stonefish is the most venomous fish known. Lying on the seabed, looking exactly like an encrusted rock, it waits for small fish and shrimps to swim by. Then with lightening speed (just 0.015 seconds) the fish opens its mouth and sucks them in.

    Stonefish reach up to about 15 inches (35 cm) in length and live in the Indo-Pacific region and northern Australian waters. This fish has 13 grooved spines sharp enough to puncture rubber, which it uses for pure defense against bottom-feeding sharks and rays. It has a spiny head with large pectoral fins, and it is bluntly built and covered with flaps of skin and wart-like bumps. To round out the camouflage, it is grayish-brown in color and blends in perfectly with the ocean floor.

    The stonefish is only dangerous if stepped on or caught. The dorsal spines project from venom glands along the back, and venom is involuntarily expelled when pressure is placed on them. In fact, the victim is the one who injures himself. It takes a few weeks for the glands to regenerate and recharge.

    Victims experience excruciating pain that lasts for hours and a tremendous swelling develops with the death of the tissues. Temporary paralysis, shock and even death may result. The severity of symptoms depends on the depth of penetration and the number of spines involved.

    You can prevent injury by wearing thick-soled shoes and treading very lightly; spines have been known to pierce through a shoe. If you are stung by a stonefish, you can immerse the area in hot water. But for severe symptoms, which include excruciating pain, weakness and paralysis, and multiple punctures, the only real treatment is the administration of stonefish antivenin – and an intravenous narcotic analgesia for pain.

    Aggression in Your Aquarium

    Aggression is one of the most common problems when it comes to keeping pet fish in your aquarium or pond. Injuries caused by aggression can lead to life-threatening bacterial, fungal and parasitic disease in your fish. You can solve this frequent cause of death and illness in an aquarium by fixing incompatibility problems.

    The solution usually requires physically separating the aggressive and submissive fish. Many fish hobbyists – especially beginners – want variety in their new aquarium. All too frequently, fish that are attractive (but maintained separately) in the pet store, won’t get along in the same aquarium or outdoor pond.

    For example, larger koi will commonly injure or even kill small goldfish. Members of the same species may even kill and eat smaller or weaker members of their own species.

    Fish that are scared but manage to survive attacks by a more aggressive fish may go without food if they spend too much time hiding under a rock or in the corner of the aquarium. The responsible pet store clerk will usually advise you on which fish will get along. Despite this advice, some aquarists still gamble and place non-compatible individuals in the same system.

    Tips for Preventing Aggression

  • Do your homework. There are plenty of books and magazine articles that discuss the behavior and habits of most pet fish species.
  • Consult with your pet store clerk or knowledgeable members of your local aquarium society.
  • Provide plenty of structure in your aquarium in the form of rocks, corals, plants and driftwood. These items can provide life-saving refuge for your smaller and more submissive pet fish.
  • Keep your fish well fed – but not overfed.
  • The External Anatomy of a Fish

    If you’re someone who’s going to care for your own fish instead of having a service maintain your tank, you need to know a little bit about fish anatomy. Here are some of the basic – and not so basic – external fish parts.

    Remember that unlike humans, whose basic form as a species is upright, fish range much more in shape, and even in which body parts they have. But, generally speaking, they’re similar enough and easy to identify.

    The Shapes of Fish

  • Fusiform. Bullet-shaped, the most common form.
  • Truncate. Shortened fusiform like a triggerfish or cowfish.
  • Serpentine. Eel or snake-like.
  • Attenuate. Long and thin like a needlefish.
  • Compressed. Disk-like to bullet-shaped and very thin, like an angelfish or butterfly fish, or very flat, like a flounder.
  • Fins

    Perhaps the most familiar parts of a fish are its fins. Fins are not much more than sheets of skin that hang from the fish and are supported by a structure of rays to offer a fish stability when jetting through the water. These rays are made from organic material that can be soft or hard. Throughout evolution, some fins have evolved into protective devices like spines, or into a bait decoy that lures the prey closer to a hungry mouth.
    The Extras

    Some fish have these various decorations and adaptations, many do not.

  • Cirri. Hair-like projections that extend from the forehead area (called the nape).
  • Fin-spine. A hard fin ray that provides not only support to the fin, but also a protective device.
  • Barbel. A whisker-like projection or projections located under the mouth of a fish.
  • Snout. The mouth-area of the fish.
  • Nape. The forehead-area before the dorsal fin begins.
  • Lateral line. A row of highly sensitive nerves aligned usually along the center of the body. These nerves help the fish detect water pressure changes and other environmental conditions.
  • Dorsal fins. These can be single or in pairs. These fins are on the top and toward the front of the body when seen from the side. Sometimes, there is a second dorsal fin behind it – if so, the plain dorsal fin gets promoted to first dorsal fin.
  • Pectoral fins. In some fish these fins, found on opposite sides of the fish behind the gill slit or close to them, have adapted to something more like arms.
  • Ventral fins. One or more fins located on the bottom of the fish when seen in profile.
  • Anal fin. A fin that is positioned very close to where the tail begins and opposite the second dorsal fin. Not all fish have this fin.
  • Finlets. Some fish, like tuna, have a small series of fins that run along behind a bigger dorsal fin.
  • Gill slits. The obvious slits that are the opening to the gills. They commonly denote where the head ends and the body begins.
  • Gill-plate cover. The “flap” of skin that covers the gills.
  • Scales. Fingernail-like plates that cover the skin of many fish. Some scales are bigger, and more obvious, than others.

    The tail fin, also called the caudal fin, can be either a single fin or forked. If it is forked, the upper and lower parts of the fork are called the upper and lower lobes, respectively.

  • Border of tail. The lower and upper parts of a tail fin that make up part of the “fan” of the tail.
  • Margin of tail. The edge of the tail fin that lies on the end or outside of the tail fin “fan.”
  • Base of tail. The boundary where the body of the fish ends and the tail fin begins.
  • Why Are Fish So Slimy?

    There is a reason that fish are so slimy, and it’s very important for the survival of fish. Slime is secreted from cells in the very outside layer of the skin. In some cases, the placement of these slime-producing cells determines what species of fish it is. The cells produce what is called a glyco-protein, which is then mixed with the water making the slimy mucus.

    Slime is important for fish to regulate many necessary body functions, including protection against parasites. Some parasites can’t attach to the scales because they are too slippery; others suffocate in the slime. Slime also acts as a band-aid, covering wounds and scrapes. Some studies indicate that slime may aid in reducing the turbulence along the body, making the fish faster in the water.

    Some fish also take advantage of their coating and lace it with a toxin. If some creature takes a bite, it may suffer the consequences and think twice the next time. Some fish form cocoons out of their slime. Parrotfish, for example, produce a mucus “tent” that completely envelopes them, which may protect them from attack. Slime can also regulate, on a molecular level, the exchanges that are made between the fish’s body and the surrounding seawater. Fish “breathe” with their gills, but they also breathe through their skin. The slime allows the necessary chemical molecules to pass between the inside and outside of the fish.

    In some aquarium fish such as tetras, barbs, sharks, rasboras and loaches among others, the skin contains a number of cells that have no opening to the outside, called “blind cells.” When these cells are broken, which occurs by a rupture in the skin, they release a fright hormone that is transmitted through the water and sends an alarm of sorts out to other fish. Suddenly these fish will dart about, sometimes ramming into the sides of the tank or decorations for no apparent reason. These substances aren’t species specific, but seem to be an alarm for all. Only a water exchange (at least 3/4 partial exchange) will solve this problem once these fright substances are in the water. They must be chemically removed and won’t be filtered out through your system.

    Several fish secrete body slime with which to feed their young. Some of it can be high in proteins and fats which is what a young growing creature requires.

    Body slime diseases and ailments should be taken seriously. Always inspect any fish that you are considering for purchase and make sure that you do not see any lacerations, swellings, or slime streaming from its body. This could indicate a sick or dying fish. Deteriorating slime and skin is very difficult to recover from.

    In some cases, parasites may have gotten to a wound. In any case, it is important to quarantine any fish you are considering to add to your community aquarium. Keep the newcomer on his own for about five days and watch him. Observe his body and watch for any indications of disease or abnormal conditions-such as streaming slime. Such diseases will quickly spread to your other fish.

    The Internal Anatomy of a Fish

    The natural history of fish is rich and diverse, going back about 468 million years more than our own. Since people usually own aquariums because they enjoy observing these creatures, knowing about their history lends a respectful appreciation for them.

    According to fossil records, fish were among the first kind of animals to appear on the planet. The earliest known is a jawless fish that presumably roamed the seabed sucking up invertebrates. This mother of fishes lived 470 million years ago, during what geologists call the Ordovician Period in Earth’s history. Hagfish and lampreys are now the only two living representatives of jawless fishes.


    The first jawed fishes appeared in the geologic record about 100 million years after jawless fish came about. Today, two major groups of jawed fish exist and they are distinguished by what sort of material their skeletons are made of – cartilage or bone.

    The cartilaginous fish include the sharks and rays and are represented by about 165 living species today. Cartilaginous fish have a skeleton made of cartilage – the firm but flexible tissue that forms your nose. This group of fish also lack true scales on the skin, the gill flap (or operculum) that covers the gill, and movable fins. Bony fishes, on the other hand, have a bone-like (or ossified) skeleton that supports the body tissue.

    Teeth and Skulls

    Almost all fish have teeth. Some have teeth that extend deep into their pharynx (called pharyngeal teeth). The esophagus of a fish usually can distend considerably to accommodate large prey. Within the skull are ear stones, also called otoliths, that form not only part of the auditory system, but also aid in the fish’s balance abilities.

    Swimming Mechanisms

    Fish have a swim bladder that allows them to maintain their position within the water with very little effort. It was believed that before fish evolved this buoyancy organ, they would have needed to swim constantly in order to maintain their depth. Many experts on fish evolution believe that the eventual development of the swim bladder allowed fish to swim slower and become more maneuverable and agile, and that these free swimming habits were accompanied by changes in body form and fin shapes, forms and function. All of this, they argue, allowed fish to occupy every possible habitat in the seas.

    Breathing Mechanisms

    Fish “breathe” water as we breath air. Our lungs make the necessary exchanges on a molecular level to keep our bodies functioning. These organs take the oxygen molecules in the air we breathe, sequester it for important cellular functions and get rid of the carbon dioxide that is our form of exhaust. In the same way, the gills of fish allow their necessary “breathing” chemistry to take place. They do this via layered membranes that make the necessary molecular exchanges as water flows across them.

    Most fish have small brains – but can be trained on a very basic level, such as eating frozen food if the usual diet of live brine shrimp isn’t available.


    Reproduction in fish normally occurs by the female producing a large amount of small eggs. These are usually externally fertilized and hatch into larvae. This is an important part of the lifecycle since during this stage, as plankton, they are swept up and dispersed by currents. They soon grow to become baby fish, called fry. Certainly, it is easy to name the exceptions to this general reproductive system.

    There are several species in which internal fertilization occurs. They have developed ingenious methods of keeping their young alive because they have no placenta, as mammals do. Other species, such as seahorses, deserve their own category. The female hands the eggs over to the male during “copulation,” who then arranges them inside the belly and fertilizes them. Several weeks later he gives birth to live seahorse fry.

    Most Are Cold-Blooded

    Most fish have stomachs although some herbivores may not – which would require a lifestyle of never-ending grazing. Usually, fish are cold-blooded, although some large, fast-swimming, open-ocean fish such as tuna give off heat as they swim and either recycle it or have a poor system of getting rid of it during the rigorous swimming exercise. Some sharks and tuna have this “warm-blooded” tendency, which is defined by the fact that their blood is warmer than the surrounding seawater.

    The diversity of fish is far greater than the species we are even capable of keeping in captivity – that we can visit in state-of-the-art aquaria. The most advanced science has yet to find a way to keep the young of a billfish such as a swordfish or marlin alive in captivity for more than a week, and no one can pinpoint the reason why.

    The wealthy sport fishing industry is obviously interested in that question and has provided means for researching such issues – but the answer remains elusive. Other species cannot live at our temperatures and pressures, surviving only in the deepest reaches and alien atmospheres of ocean-space. The species alive in your tank may not be as exotic, but they represent several hundreds of millions of years of history and survival on a planet we’ve existed on only for 2 million years.

    How Do Fish Breathe?

    Inquisitive minds want to know. How can fish breathe underwater if there is no air? Fish and humans resemble each other in many ways. We have digestive, circulatory and nervous systems similar to those of other vertebrates. And fish need oxygen just like we do. But when we spend time underwater we must carry air with us.

    What really makes a fish different is the respiratory system. The water that surrounds a fish contains dissolved oxygen in a ratio of about 5 ml per liter. Therefore they must have a system that can extract that small amount of oxygen out of the water and into their bloodstream. They use their gills for this, which are found between the mouth and the beginning of the gut or food tract called the pharynx.

    The process starts with the fish’s mouth where he takes in water. When a fish opens and closes his mouth, he is pumping water back through the gills – his own way of breathing. The gills contain thousands of tiny capillaries (blood vessels), so as water passes over the gills, oxygen is absorbed directly into the bloodstream.

    Bony fishes, which are the most common, have an effective pumping system that involves the mouth and the outer cover of the gills, called the operculum. When the fish’s mouth opens, the operculum closes, drawing water into the mouth. When the fish closes his mouth, the operculum opens and allows fresh water to cross the gills. Oxygen passes through the thin wall of the gills and into the blood.

    Other fish, such as the tuna, have less effective pumping systems and must swim constantly to keep fresh oxygenated water flowing over their gills. They usually swim with their mouths partly open.

    Their World of Water

    Water temperature can greatly affect the concentration of free oxygen in the water. As water temperature increases the free oxygen concentration decreases. Stagnant or poor quality water also contains less oxygen. Since some fish require more oxygen than others, this helps to explain why some fish thrive in specific habitats. For example, trout prefer the cool waters of the northern streams because they are quite active and the cool water retains dissolved oxygen more readily. Carp are sluggish and don’t require as much oxygen; they can thrive in warm, relatively stagnant water, such as ornamental ponds.

    The surface of the water where it comes into contact with the atmosphere tends to hold a higher content of oxygen. Goldfish can survive in non-aerated aquariums because they spend most of their time at the surface.

    The Home Aquarium

    Just as fish extract oxygen through their gills, they also produce carbon dioxide as waste and discharge it into the water. To be certain that there is enough oxygen in the water for the fish to “breathe,” the carbon dioxide needs to be replaced by oxygen. This is called aeration.

    Aeration is the process by which oxygen is transferred from the air to the water. In the aquarium aeration is accomplished by breaking the surface of the water or by creating turbulence in the water. This is relatively easy to achieve in a freshwater tank, a bit more difficult in a saltwater aquarium.

    In your fish tank, aeration happens in a few ways:

  • First, on the surface of the water in the tank, dissolved gases are released into the air and exchanged for oxygen. The greater the surface area, the greater the amount of oxygen taken in, which is why fancy narrow tanks or fish bowls with very little surface area can only support a couple of fish.
  • Second, when water passes through a power filter and is exposed to the air before it’s pumped back into the tank (which is the case with most power filters) the water is being aerated.
  • Finally, an air pump connected to a bubbler in the tank – an airstone or one of those little bubbling divers – provides air from the bubbles as they burst in the tank. These are all methods of mechanical aeration. Aeration also occurs when live plants release oxygen as they grow.

    Since small fresh water tanks – 10 gallons and less – have little surface area, adding aeration through an airstone can be helpful. In larger fresh water tanks outfitted with power filters, the movement of the water caused by the filter will usually provide enough oxygenated water without an airstone, although the stream of bubbles from the stone can be pleasant to watch and certainly won’t be doing any harm.

    Our fish are not so different from us – they see, taste, smell, hear and breathe – but they do it in their own underwater world. Understanding how fish utilize oxygen and rid their bodies of carbon dioxide, all underwater, will help you provide them with the best care.

  • Trouble in the Aquarium – When Fish Fight

    Aggression is one of the most common problems when it comes to keeping pet fish in your aquarium or pond. Injuries caused by aggression can lead to life-threatening bacterial, fungal and parasitic disease in your fish. You can solve this frequent cause of death and illness in an aquarium by fixing incompatibility problems.

    The solution usually requires physically separating the aggressive and submissive fishes. Many fish hobbyists – especially beginners – want variety in their new aquarium. All too frequently, fish that are attractive (but maintained separately) in the pet store, won’t get along in the same aquarium or outdoor pond.

    For example, larger koi will commonly injure or even kill small goldfish. Members of the same species may even kill and eat smaller or weaker members of their own species.

    Fish that are scared but manage to survive attacks by a more aggressive fish may go without food if they spend too much time hiding under a rock or in the corner of the aquarium. The responsible pet store clerk will usually advise you on which fish will get along. Despite this advice, some aquarists still gamble and place non-compatible individuals in the same system.

    Tips for Preventing Aggression

  • Do your homework. There are plenty of books and magazine articles that discuss the behavior and habits of most pet fish species.
  • Consult with your pet store clerk or knowledgeable members of your local aquarium society.
  • Provide plenty of structure in your aquarium in the form of rocks, corals, plants and driftwood. These items can provide life-saving refuge for your smaller and more submissive pet fish.
  • Keep your fish well fed – but not overfed.