214 IDEAL HOME LIFE
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 aquarist. Temperature, which is such an important factor in the natural pond, can easily be controlled indoors within the limits of variation which are likely to affect seriously the inhabitants of the aquarium. Similarly the light factor usually offers but little difficulty, 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 necessary to grow plants in the aquarium. It is a well-known fact that in manufacturing their own food from simple substances plants give off oxygen as a waste product. This process is accomplished by the green matter of the plant, a special kind of protoplasm known as chlorophyll. In the submerged plants of the aquarium the oxygen passes off directly by absorption into the water. The fishes are thus supplied with oxygen given off by the plants as waste substance.
Having absorbed the oxygen into the blood by means of