MAKING OUR HOME BEAUTIFUL 215
the gills, the fishes combine it with the carbon of the food to obtain energy, and, in the process of respiration, give off to the water quantities of carbon dioxide, or carbonic-acid gas, as a waste substance. This gas, composed of carbon and oxygen, is absorbed by the plants and the carbon used in the process of starch making, while the oxygen is returned to the water again as a waste substance. Thus the animals and the plants of the aquarium are mutually benefited, each supplying something that is required in the life processes of the other.
Plants, however, are able to manufacture starch, and consequently absorb carbon dioxide and release oxygen, only when they are exposed to sunlight. It follows then that on dark days the plants have less capacity for aeration than on bright days, and that they yield more oxygen in sunny windows than in dark corners. Moreover they can make starch and consume carbon dioxide and yield oxygen only during the daytime. Further than this, they consume a small amount of oxygen in their own respiration both day and night, so that at times when they are not engaged in starch-making they tend to consume a part of the oxygen of the aquarium, although in a night they can use only a small portion of that thrown off during the day. If the water of the standing aquarium is supplied with an excess of oxygen during the day, a considerable amount of the oxygen will remain in solution in the water and aid in proper aeration throughout the night.
It is evident then that an aquarium well stocked with plants will support a larger quantity of animal life during the day and in bright weather than it will at night or on dark days* The animal life of the standing aquarium must therefore be regulated to meet the poorest rather than the best conditions of oxygen production by the plant life.
Temperature also affects the rate of starch-making and consequently of oxygen elimination, as the protoplasm of the plant is more active in a higher than a lower temperature. However, the fishes are also less active in colder water and consume less oxygen, so that these factors balance each other