You can sometimes buy aquarium cement or prepared putty at a “gold fish” store.  This you will need to putty in the glass.  If you cannot buy it, make it yourself from the asphalt varnish and whiting.  Be sure that the paint and putty of an aquarium is thoroughly dry before you fill it with water.

Perhaps the most satisfactory way to study fish and insect life in water is to use all glass boxes and globes.  So many kinds of fish and insects are natural enemies, even though they inhabit the same streams, that they must be kept separate anyway.  To put them in the same aquarium would be like caging up two game roosters.  If we were studying the development of mosquitoes, for instance, from the larvae or eggs to the fully developed insect, we should not get very far in our nature study if we put them in an aquarium with fish.  A fish will soon make short work of a hundred mosquito wigglers just as a large frog will eat the fish, a snake will eat the frog and so on.

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 near-by 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.