Fish Speak: Common Fish Terms Defined

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Any time you pursue a new interest, it is important to learn the terminology used by experienced hobbyists, and it’s best to do it before you make any decisions or purchases. To successfully set up and maintain an aquarium and its residents, a basic knowledge of aquarium and fish terms is important. It will help you converse with other enthusiasts, aquarium store personnel, and understand books and references. This article will define basic terms of environment, water chemistry, anatomy, disease and equipment. As your experience grows, you will want to consult more specialized references.


Everyone knows that fish are found all over the world and in a variety of climates. Fish from similar climates and water conditions often live together happily. Some should not be combined due to their extreme differences in natural environments.

  • Freshwater. Freshwater fish are those species found in bodies of water such as lakes and rivers. The water of their natural habitats contains no or very little dissolved salt.
  • Saltwater. This term relates to the fish that are natural to seawater and oceans. The water contains appreciable amounts of dissolved salt. Saltwater fish must have their natural environment duplicated in the aquarium.
  • Marine. Marine is a term that defines the ocean environment and its inhabitants. It is often used interchangeably with “saltwater.”
  • Brackish. Brackish refers to the type of water occurring where freshwater begins to contact ocean water. It has a higher salt content than freshwater, but lower than seawater. It can often look very dark and murky.
  • Tropical Fish. These fish come from warm climates where water temperatures range from 68 to 80 degrees Fahrenheit. There are both tropical freshwater and tropical saltwater fish.
  • Temperate. Temperate fish prefer water of cooler temperatures, usually between 45 to 65 degrees F.

    Looking at an aquarium with no fish isn’t that entertaining but you would be surprised at the number of chemical and biological reactions that are occurring every second. Water is very dynamic and many factors influence its quality. Water quality is the number one concern for any aquarium.

  • pH. pH is a measure indicating whether water is acidic, alkaline or neutral. The pH scale ranges from 1 to 14, with 1 to 6 considered acid, 7 considered neutral, and 8 to 14 considered alkaline. The pH of an aquarium is influenced by a number of factors such as organic waste, gravel, natural decorations, medications, etc. Fish have evolved to live in a wide range of pH, some having more sensitive needs than others.
  • Hardness. Hardness is a term that describes the amount of dissolved mineral salts in the water. Calcium and magnesium make up the majority of these salts and are different than the sodium chloride salt in seawater. Hardness values are compared on a scale of 0 to 400. Zero to 50 is considered soft water, 50 to 200 slightly hard and 200 to 400 is hard water. Because of the ongoing chemical reactions between water, calcium and magnesium, the total hardness of the water can influence its pH and other aspects of water quality.
  • Nitrogen Cycle. The nitrogen cycle is the process by which organic waste is broken down by beneficial bacterial existing in your aquarium. These bacteria are the cornerstone of a healthy aquarium.
  • Salinity. Salinity refers to the amount of dissolved sodium chloride in the water. Salinity varies in different bodies of water and can be measured as specific gravity.
  • Specific Gravity. Specific gravity compares the amount of salt in the water to that of freshwater. Freshwater, containing no salt, has a value of 1.000. Saltwater will range in value from 1.015 to 1.030 in most marine environments.

    Fish have a unique anatomy and it helps to know a few things about their parts. Many times, you can make health determinations by looking at their general appearance, and also recognize disease in those organs you can’t see.

  • Gills. Fish breathe by extracting oxygen through their gills. They should look a nice healthy reddish pink. A variety of parasites attach themselves to the gills so it’s a good idea to check your fish’s gills for problems before you buy.
  • Opercula. The bony plate covering the gills is called the opercula. It is a great way to monitor respirations. Counting how many times the opercula move in and out equals the number of respirations. Rapid respirations often mean stress, poor water or disease.
  • Lateral Line Canal. The lateral line is a series of small pits under the skin that are responsible for detecting sound transmitted as pressure and may have a weak electrical function in some species.
  • Swim Bladder. The swim bladder functions as an organ of buoyancy and can be inflated or deflated with air. Diseases of the swim bladder are often noted in a fish’s erratic swimming behavior.

    Fish suffer from a variety of diseases cause by parasites, viruses, bacteria and fungi. Many are common and you should learn to recognize infected individuals to avoid the purchase of a sick fish or remove one from your show tank

  • Ich. Ich is the short form of the name of an external parasite named Icthyophthirius multifilis. Most everyone who buys or raises fish has seen a case of ich. Fish develop small raised white spots that are irritating and cause itching.
  • Fin or Tail Rot. Fish fins should look strong and healthy with regular margins and no defects or sores. Fin rot makes fins look ragged, and you may see small hemorrhages or bruises in the fins. This condition is most often caused by a bacterial infection that can be fatal. Affected individuals should be removed and treated in a hospital tank.
  • Swim Bladder Disease. The swim bladder can become infected or inflamed and cause an erratic swimming pattern. Fish may be unable to control their buoyancy and even swim upside down.

    The number and types of tanks and equipment available for aquarium set up is limited only by your budget. Initially, start with a simple tank with basic equipment until you learn more about your hobby and specific needs of your fish. Basic supplies will include:

  • Tank. Tanks range in size from 1 gallon to several thousand gallons. A good beginners tank is 10 to 30 gallons. Buy a model that is long rather than tall, it will give the fish more surface area and swimming room.
  • Filter. This is the system that purifies the water and has the most variability in type and expense. An under gravel filter is a good basic start. It sits in the tank under the gravel and runs by a small pump. An external filter cleans the water through biological, chemical and mechanical means. Buy a filter large enough to accommodate your tank.
  • Heater. If you are keeping tropical fish, you will need a way to maintain a constant tank temperature. Heaters should be large enough to accommodate for the number of gallons of water your tank holds.
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