Many people enjoy watching colorful fish dashing around their aquarium during the daylight hours. Hues of oranges, yellows and reds, purples, blues and greens dart about the tank.
But few people know about the quirky night rituals of some fish that can make a nocturnal tank rewarding and interesting. Hiding out for most of the day, many of these "swing-shift" fish are active only at night. Coral polyps, for example, are usually closed up during the daylight hours. At night, they open up, extending their little arms into the water to use the current to trawl for prey.
The most active time for nocturnal fishes is during the early evening and morning hours, when they feed. Grouper, lionfish and eels, are among the species that are active mostly at night, and about four families of fish are almost strictly nocturnal.
You can keep a nocturnal tank by creating a cave-like environment and keeping the tank away from a bright window or area. Use coral rocks and try to set up the hiding spaces so you still may observe your pets under low light. You don't want nocturnal conditions to exist all the time unless you have species that are usually found at much greater depths than those typically at coral reefs.
The only main difference in setting up a nocturnal tank is the lighting. You must pay attention to the amount of light you will expose your creatures to. Obviously, the ambient light during the day will be the period of lowest activity in your tank. To see the active creatures without disturbing them, you must use a red light.
To create a thematic tank such as a nocturnal, reef or cold-water aquarium, you should already have some experience with saltwater systems and understand how to fulfill the necessary conditions that the species require. Some of the fish suggested for the nocturnal tank should be kept only be experienced hobbyists because they can be dangerous when mishandled or carelessly fed.
Many nocturnal fishes are hues of faded red and reddish yellows because this color is the first to fade as one goes deeper into the water column. They also commonly have big eyes and are fairly shy. Corals can be kept in a nocturnal tank only if you make sure they get enough light during the day to appease their zooxanthellae (such as coral polyps). Soft corals may also be invertebrates to consider for the nocturnal tank. Many of these are used to lower light levels and would thus survive a tank that you've purposefully sheltered from normal daylight.
Some species of fish to consider for a nocturnal tank include grouper, marine eels, squirrelfish, soldierfish, the rare pineconefish, and cardinalfish. Another group of fish that are ideal for a nocturnal tank are the Beryciformes. These fish are mainly lie-in-wait predators, waiting for prey to swim within range before they pounce. They must be fed live foods and cannot be kept with small invertebrates or fishes smaller than itself.
Nocturnal fish have many interesting habits. Some fish, like tangs for example, blow a mucus bubble around themselves at night. And surgeonfish find a nook and lodge themselves in tight by propping up their spine against the reef, making it impossible to dislodge them without a struggle.