2. The aquarium must be ventilated. Its top should be broad and open. Every little fish, snail and insect wants air, just as every boy and girl wants it. A certain quantity of air is mixed with the water, and the creatures must breathe that or come to the surface for their supply. How does Mother Nature manage the ventilation of her aquaria,—the ponds and streams? The plants furnish part of the air, as we have said. The open pond, whose surface is ruffled by every passing breeze, is constantly being provided with fresh air. A tadpole or a fish can no more live in a long-necked bottle than a boy can live in a chimney.
3. The temperature should be kept between 40° and 50° Fahr. Both nature and experience teach us this. A shady corner is a better place for the aquarium than a sunny window on a warm day.
4. It is well to choose such animals for the aquarium as are adapted to life in still water. Unless one has an arrangement of water pipes to supply a constant flow of water through the aquarium, it is better not to try to keep creatures that we find in swift streams.
Practical experience shows that there are certain dangers to guard against,—dangers which may result in the unnecessary suffering of the innocent. Perhaps the most serious results come from overstocking. It is better to have too few plants or animals than too many of either. A great deal of light, especially bright sunlight, is not good for the aquarium. A pond that is not shaded soon becomes green with a thick growth of slime or algæ. This does not look well in an aquarium and is likely to take up so much of the plant-food that the other plants are "starved out." The plants in the school-room window may provide shade for the aquarium, just as the trees and shrubs on its banks shade the pond. If we find green slime forming on the light side of our miniature pond, we should put it in a darker place, shade it heavily so that the light comes in from the top only, and put in a few more snails. These will make quick work of the green slime, since they are fond of it, if we are not.
Fig. 56. A rectangular glass aquarium.
Some of the most innocent-looking "water nymphs" may be concealing habits that we can hardly approve. There are some which feed on their smaller and weaker neighbors, and even on the members of their own families. We know that such things go on in nature, but if we wish to have a happy family we must keep the cannibals by themselves.
After an aquarium has been filled with water and the inhabitants well established, it is not necessary to change the water, except in case of accident. The water that is lost by evaporation has to be replaced. It should be poured in gently in order not to disturb the water and destroy its clearness. If a piece of rubber tubing is available, a practical use of the siphon can be shown and the aquarium replenished at the same time. It is a good plan to use rain water, or clear water from a pond, for this purpose.
A piece of thin board or a pane of glass may be used as a cover to keep the dust out of the aquarium. This need not fit tightly or be left on all the time. A wire netting or a cover of thin cotton net would keep the flying insects from escaping, and it might be tied on permanently. Dust may be skimmed off the top of the water or may be removed by laying pieces of blotting paper on the surface for a moment.
If any of the inhabitants do not take kindly to the life in the aquarium, they can be taken out and kept in a jar by themselves—a sort of fresh air and cold water cure. If any chance to die they ought to be removed before they make the water unfit for the others. Bits of charcoal in the water are helpful if a deodorizer or disinfectant is needed.