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. [Illustration: _Fig. 57. A home-made aquarium._] Experience, the dear but thorough teacher, is of more value to every one of us than many rules and precepts. Nothing can rob us of the pleasure that comes of finding things out for ourselves. Much of the fun as well as much of the success in life comes from overcoming its difficulties. One must have a large store of patience and courage and hopefulness to undertake the care of an aquarium. After it is once made it is less trouble to take care of than a canary or a pet rabbit. But most things that are worth doing require patience, courage and hopefulness, and if we can add to our store of any of these by our study of life in an aquarium we are so much the better for it. Two kinds of aquaria will be found useful in any school. Permanent ones--those which are expected to continue through a season or through a whole year if the school-room is warm enough to prevent freezing; and temporary ones--those which are for lesson hours or for the study of special forms.