1. The equilibrium between plant and animal life must be secured and maintained. It is probable that an aquarium in an elementary school is mainly used for the study of animal life; but animals do not thrive in water where no plants are growing. Nature keeps plants and animals in the same pond and we must follow her lead. The plants have three valuable functions in the aquarium. First, they supply food for the herbivorous creatures. Second, they give off a quantity of oxygen which is necessary to the life of the animals. Third, they take up from the water the harmful carbonic acid gas which passes from the bodies of the animals. Just how the plants do this is another story. [Illustration: _Fig. 55. A museum-jar aquarium. (More animal life would make a better equilibrium._)] 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. [Illustration: _Fig. 56. A rectangular glass aquarium._]