When selecting fish for your aquarium, it is important to consider whether they are normally active by day or night. This will affect not only your enjoyment of your pets, but also the choice of tank mates that you might wish to include in the aquarium.

Nocturnal fish such as freshwater eels need a place to hide during the daytime. If denied this, they may become stressed and will languish in captivity. However, eels and other such species change radically at night, and their level of activity will surprise you. Most catfish, for example, spend the night swimming about looking for food. These nighttime wanderings may disturb shy, diurnal fish and prevent them from resting properly, thereby impairing their health. Many fish that are innocuous by day morph into quite aggressive predators as night falls. Be sure that you are aware of and understand all such predatory propensities before deciding which species to keep. For example, small octopus and moray eels are usually quite content to spend the day secluded in a favorite retreat. If this is the only time that you observe them, you might mistakenly believe such animals to be compatible with your other fish. However, at nighttime they undergo quite a change and will quickly devour smaller neighbors.

Be sure that diurnal fish have appropriate retreats, as many species use them at nighttime. Certain fish, such as a parrotfish, do not use shelters by day, when they swim about quite like most other fish. However, at night, they habitually secret themselves within caves and crevices, and they may become stressed if prevented from doing so in captivity.

Bear in mind that many nocturnal fish will not feed during the day. Some, as is pointed out in the various species accounts (Chapter 7 and Chapter 10), will forego their nocturnal habits once they adjust to captivity. Bullhead catfish, moray eels, and, occasionally, octopus fall within this category. Many fish, are, however, strictly nocturnal and must be fed during those times when they are normally active if they are to do well in the aquarium.

Keep in mind that the cautions that have been mentioned concerning housing nocturnal fish with diurnal species apply to active and sessile fish as well. For example, most typical “sit and wait predators,” such as anglerfish, will be greatly disturbed if forced to remain in close proximity to vigorous, actively swimming fish and will usually not obtain enough food when aggressive feeders are present.