402 NATURE STUDY AND LIFE 



How often does the water require changing ? This is 

 the question often asked by those who wish to start an 

 aquarium. The answer is : ''Not once a year, if the ani- 

 mals and plants and light are properly balanced and regu- 

 lated." In doing this it is best to use the plants and 

 animals that live in still water, in ponds and quiet pools, 

 rather than those characteristic of the running streams. 

 Aquaria with running water are more difficult to manage 

 and are expensive in use of water. They are, moreover, 

 a constant menace as to flooding the building, and cannot 

 be recommended for school use. 



What causes the water to become cloudy or milky ? 

 This is the next troublesome question, and with fifty chil- 

 dren all anxious to feed the fishes something, it would 

 seem that every school aquarium must be in this condi- 

 tion all the time. But one of the chief uses of an aqua- 

 rium is to make the children careful and thoughtful, and as 

 experience has shown, they readily learn that giving more 

 food than the fishes and other animals will eat clean, gen- 

 erally before it sinks to the bottom, is the quickest way to 

 make the water foul and, possibly, to kill their pets. We 

 shall learn more of this when we study about bacteria in 

 the water. Fishes, and aquatic animals in general, should 

 not be fed oftener than once a day, and then only so much 

 as will be eaten clean. If mistakes are made in their eager- 

 ness to feed bountifully, the children who do it should be 

 asked to remove all surplus food with a siphon or dipping 

 tube before it has had time to decay. 



What animals can be safely kept together in the aqua- 

 rium is another frequent question. The general rule of 

 keeping predaceous species by themselves, except while 



