While a fish lying on the bottom of your aquarium may be doing nothing more than napping, it may also be indicating an imbalance in the fish's environment. To keep your fish healthy and contented, there are several things you should monitor regularly in your aquarium. If you get lax, fish lying on the bottom of the tank could be telling you to fix the water or call his next of kin.
If you notice your fish gasping for air at the water's surface or lying for long periods of time on the bottom of the tank -- especially if the gills have a reddish or purplish look or the fins appear to be streaked with red -- your tank may have a high ammonia content. Checking the water's pH value and, if the pH value is over 7, replacing half of the water should remedy the problem. If the ammonia content remains too high, consider using an ammonia neutralizer. Too many fish in a tank, overfeeding and failing to maintain the water on a consistent basis can lead to this problem.
You may notice fish lying on the bottom of the tank more if the water is not aerated. This is likely to be a problem if the water is too warm or too cool. While cool water cannot absorb much oxygen, and water that is too warm will release more into the atmosphere. Keeping the temperature uniform -- in the upper 70s, Fahrenheit -- and circulating the water mechanically will help ensure optimum oxygen content.
Stress and Trauma
Some species of fish will spend more time lying on the bottom of the tank when they become stressed or scared. If your fish is lying on the bottom and breathing rapidly, stress may be the problem. Stress can lead to swim bladder disease in some species, which can manifest in a fish's off-kilter float or in the fish lying at the bottom or top of the aquarium. Physical trauma such as an attack by another fish or rapid changes in water quality can lead to swim bladder disease.
Sleep vs. Exhaustion
Most fish sleep at the bottom of an aquarium, especially if the environment is dark. If they are sleeping, their breathing will be regular. If you leave aquarium lights on around the clock, the fish will probably keep swimming until they become exhausted, then lie on the bottom of the tank for extended periods during daylight hours. Exhausted fish are more susceptible to illness.