16 NEW YORK AQUARIUM NATURE SERIES 



without them. Nearly all colleges and universities have some 

 means of maintaining aquaria, usually of the balanced sort, 

 while a few even possess facilities for the storage and circula- 

 tion of sea water in larger tanks. 



Naturally, larger aquaria have the advantage of supporting 

 a larger and more varied stock, but it should be borne in mind 

 that for scientific as well as for other purposes, the proper 

 adjustment is of far greater value than mere space or variety of 

 life. In the high-school, grade-school and even in the kinder- 

 garten, balanced aquaria have found a place where they encour- 

 age nature study among the children. The New York Aquarium 

 has eciuipped hundreds of these for various schools in New York 

 City. 



THE MEANING OF BALANCE. 



The factors which govern life in the balanced aquarium are 

 the same as those which obtain elsewhere in nature, with the 

 important difference that certain of them are under control. In 

 fact we may consider the aquarium as a miniature pond in 

 which the conditions of food, temperature and aeration are under 

 the control of the operator. In the natural pond the variations 

 of temperature alone are sufficient to produce important cycles 

 in the balance and in the life of the organisms. 



To secure and maintain a balance in the indoor aquarium is 

 the most important problem which confronts the amateur aquar- 

 ist. Temperature, which is such an important factor in the 

 natural pond, can easily be controlled indoors wathin the limits 

 of variation which are likely to affect seriously the inhabitants 

 of the acjuarium. Similarly the light factor usually offers but 

 little difliculty, and food can easily be added in the necessary 

 quantities. 



The problem of aeration is more difficult. In the natural 

 pond, with its large surface ruffled by the breeze, this takes care 

 of itself, since a sufficient amount of oxygen can be absorbed 

 from the air to supply all the animals that can find food within 

 its waters ; but in the narrow limits of the aquarium, with its 

 restricted surface, comparatively greater depth and the absence 

 of any agitation of the water, the absorption of oxygen at the 

 surface does not take place with sufficient rapidity to sustain 

 much animal life. 



To supplement the surface absorption of oxygen, it is nec- 

 essary to grow plants in the aquarium. It is a well known fact 



