FEESH-WATEE AQtTAEIA. 



8m. 



acid gas supplied by the fish, using the carbon for the con- 

 struction of their tissues and fibres, and liberating again the 

 oxygen for the use of the animal life within the aquarium; or, 

 to put it plainer, the fish breathe out carbonic-acid gas and 

 breathe in oxygen. The snails, which should always be present 

 in the aquarium, find their food in the confervoid growth and 

 decaying vegetable matter. They thus not only remove that 

 which otherwise would be injurious to the inmates of the water, 

 but they, as also do the fish, convert a part of what they eat 

 into food for the plants. 



The proof that this much- desired 

 result is attained is seen in the 

 apparent health and happiness of 

 the animals and the sparkling clear- 

 ness of the water. But to arrange 

 matters thus will require a little 

 experience. The plants, however 

 numerous they may be in the aqua- 

 rium, will not alone supply sufficient 

 oxygen. For the principal duty of 

 the vegetation is, as it has been 

 said, to decompose the carbonic- 

 acid gas expired by the animals, 

 absorbing the carbon into their own 

 substance and setting free the 

 oxygen for the use of the fish; but 

 the oxygen must be chiefly drawn 

 from the atmosphere which comes in contact with the surface 

 of the water. Therefore it wUl be seen that the shape of the 

 aquarium is a very important matter; and that this is the 

 case is proved by a simple experiment. If a wide-mouthed 

 bottle be filled with water, and an equal quantity of water be 

 poured into a shallow dish or pan about 2in. deep, and three 

 or four minnows be placed in each vessel, it will be seen that 

 while the fish in the pan remain apparently weU, those in 

 the bottle will, after having ascended to the surface of the 

 water, die. 



Fig. 1. 



The reason that the fish in the one case die and in the 



