18 NEW YORK AQUARIUM NATURE SERIES 



substance. This gas, composed of carbon and oxygen, is ab- 

 sorbed 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 aquari- 

 um are mutually benefited, each supplying something that is 

 reciuired in the life processes of the other. 



Plants, however, are able to manufacture starch, and con- 

 setiuently 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 o\vn respi- 

 ration 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 aciuarium is supplied with an excess of oxygen dur- 

 ing 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 

 v/ill 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 regu- 

 lated to meet the poorest rather than the best conditions of oxy- 

 gen production by the plant life. 



Temperature also affects the rate of starch-making and con- 

 sequently 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 and temperature 

 does not especially affect the aeration of the well-balanced aquar- 

 ium. 



THE AQUARIUM TANK. 



Undoubtedly the best kind of a receptacle for the beginner 

 is the oblong, straight-sided aquarium with metal frame, glass 

 sides and slate or soap-stone bottom. The medium sizes holding 

 from eight to ten gallons, up to twenty gallons, will be the best 

 lor the beginner. The smaller sizes are more difficult to balance 



