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Overview Studying fish behavior is both a fascinating and very practical endeavor for the fish keeper. In addition to opening one’s eyes to the amazing diversity of fish lifestyles and behavioral adaptations, research concerning your pets’ natural behaviors will also lead to much greater success in keeping and breeding them. The primary motivation behind animal behavior is survival - breeding, finding a suitable environment, obtaining food and avoiding predators (yes, human behavior often seems to break this rule!). Of course, even the best-designed aquarium is a poor mimic of nature, and so captive fishes must modify their behaviors - sometimes so much so that the actual purpose or function of what the fishes are doing will be lost on us. However, with experience, you should be able to see the relationship that the captive behavior has to its natural counterpart. A wonderful aspect of the study of fish behavior is that it will provide a lifetime of surprises – there is far too much for any one person to know, and new facts emerge, quite literally, on a daily basis. With careful observation and research, you may well be able to discover something new. Today I will focus on just one facet of behavior - the time at which fishes are active, and how this affects their welfare in the aquarium.
Nocturnal vs. Diurnal When selecting fishes for your aquarium, it is important to consider whether they are diurnal (active by day), nocturnal (active by night) or crepuscular (active in the dim light of evening and early morning). This will affect both your enjoyment of your pets and the composition of species that you might wish to include in the aquarium.
Nocturnal fishes such as the fire eel, eyrthrotaenia, and other freshwater eels, need a place to hide during the daytime. If denied this, they may become stressed and will languish in captivity. However, eels, catfishes and other species that barely move by day change radically at night, and may engage in a surprising degree of activity. These nighttime wanderings may disturb diurnal fishes and prevent them from resting properly, thereby impairing their health.
Unlike the always ready-to eat oscar, AstronotusCichlids, nocturnal fishes often appear placid by day and morph into quite aggressive predators only as night falls. For example, octopuses, snowflake moray eels, Echidna nebulous, and similar species are usually quite content to spend the day secluded in a favorite retreat - easily tricking the novice into believing that they are compatible with their tank-mates. At nighttime, however, they undergo quite a change, and will quickly devour smaller neighbors.
Many diurnal fishes, such as the princess parrot fish, Scarus taeniopterus, and related species, swim about actively in daylight but secrete themselves within caves at night. Again, if you see them only during the day, you may miss such points and fail to provide for their needs shelter-sleeping species become stressed if forced to remain in the open (at night).
Please bear in mind also that many nocturnal fishes will not feed during the day. Some, including most catfishes and moray eels, will forego their nocturnal habits once they adjust to captivity. Many, however, (i.e. the fossil catfish, Heteropneustes fossilis)remain strictly nocturnal even after years in captivity, and must be fed at night if they are to thrive.
Just as nocturnal species may unsettle diurnal fishes at night, actively swimming fish (by day or night) may stress species that are largely sessile (fish which move about only occasionally).Most typical "sit and wait predators," such as the various anglerfishes, will be greatly disturbed if forced to remain in close proximity to vigorous, mobile species (also, bottom-feeding predators usually do not obtain enough food when kept with surface-feeding fishes).
Observing Your Tank The best way to observe nocturnal fishes, and to see how diurnal fishes behave at night, is by utilizing a bulb designed specifically for nighttime aquarium viewing. Be sure to invest in such bulbs, as they will open up an entirely new world of fascinating observations and learning opportunities for you. If you plan to focus on nocturnal fishes, you may wish to consider a complete reverse light cycle, in the manner of zoo exhibits for nocturnal creatures. If a dimly lit room is available, you can leave the night-viewing bulbs on during the day, and give the fishes their “daytime” at night. This may give you more time to observe your nocturnal fishes – unless, of course, you are yourself "nocturnal"! I have been able to learn a great deal about a number of animals, both at home and while working at zoos, in this manner. Be sure to research your fishes’ natural history so that you can provide a day/night cycle of the proper length.