Rectangular glass boxes such as are commonly used for aquaria cost less than a dollar per gallon capacity. Goldfish globes cost about the same. White glass round aquaria are much cheaper and those made of greenish domestic glass are the cheapest of all, a glass tank holding eight gallons costing but two dollars. [Illustration: A self-sustaining or balanced aquarium] Any transparent vessel capable of holding water, even a Mason jar will make an aquarium from which a great deal of pleasure may be derived. The old way of maintaining aquaria in good condition required a great deal of care and attention. The water had to be changed at least once a day if running water was not available, and altogether they were so much trouble that as a rule owners soon tired of them. Modern aquaria are totally different. By a proper combination of fish and growing plants we can almost duplicate the conditions of nature and strike a balance so that the water need never be changed except when it becomes foul or to clean the glass. These are called “self-sustaining” aquaria and they are the only kind to have unless we can furnish running water from a public water supply. Self-sustaining aquaria are very simple and any boy or girl living near a brook can stock one at no expense whatever. The method is as follows: First cover the bottom of the aquarium with a layer of sand and pebbles to a depth of about two inches. Then plant in the bottom some aquatic or water plants that you have collected from a nearby lake. Any kind of water plants will do—the kind of plants boys always call seaweed, even a thousand miles from the sea. In collecting the plants, choose small specimens and obtain roots and all. If you can find it, the best plant is fanwort. Other good kinds are hornwort, water starwort, tape grass, water poppy, milfoil, willow moss, and floating plants like duckweed. Even if you do not know these by name they are probably common in your neighbourhood. Fill the tank with clean water. That taken from a spring or well is better than cistern water. After two or three days, when the plants seem to be well rooted, put in your fish. You may keep your aquarium in a light place, but always keep it out of the sun in summer and away from the heat of a stove or radiator in winter. The nature student will not attempt to stock up his aquarium immediately. He should always leave room for one more fish or bug. One year I started with a lone newt and before the summer was over I had thirteen sunfish, pickerel, bass, minnows, catfish, carp, trout, more newts, pollywogs or tadpoles, five kinds of frogs, an eel and all sorts of bugs, waterbeetles and insects. I soon found that one kind of insect would kill another and that sometimes my specimens would grow wings over night and fly away. But to learn these things, even at our own disappointment is “nature study.” If we knew it all in advance, we would not have much use for our experimental aquarium.