If a green film begins to gather on the side of the tank that is most exposed to the light, it should be cleaned away every day, and the sides of the glass polished carefully. A small piece of clean sponge tied on the end of a stick will answer the purpose very well, and, if used daily, you can keep the glass clear with very little trouble; but if the scum is neglected and left to accumulate, you will find it almost impossible to remove it from the glass even by hard scouring.
It is best to have only small fish in your aquarium, and for this reason trout are not desirable. Although very beautiful and intelligent, they grow so rapidly that they are likely to become in a short time too unwieldy for your tank. Goldfish and minnows are very good, and the common little sunfish or “pumpkin-seed” is excellent.
You must keep careful watch over the fish in your aquarium, and if any one of them appears to be sick he should be removed at once, very gently, with the hand-net, and placed in fresh water, where he will often recover. If, however, the little sufferer is doomed to die, it is better not to run the risk of his doing so among his healthy companions. It is best always to have a hospital for your sickly pets, and as soon as one of them, whether a fish or a bird or any animal, shows signs of ill health, he should be taken away from the others and placed by himself.
Certain varieties of snails live well in fresh water, and will be found useful in clearing away the green film that is almost certain to collect on the side of the glass; but you must be careful or they will devour your plants as well; and if your tank is very small it is hardly worth while to try to keep them.
Water-beetles and water-spiders also thrive well, and their habits are most interesting to watch; but water-beetles fly by night, and unless you are careful to cover your tank you are likely to discover some morning that a number of your tenants have taken French leave.
You must be careful not to overstock your aquarium, for your fish will not thrive if they are overcrowded. Remember, also, that heat and dust are fatal to your pets. The water must be kept clean and cool at all times, and all foreign matter and every particle of decaying vegetation should be removed immediately.