536 



Our Surroundings 



aquarium and life will go on even if the aquarium is sealed tight. 

 We have learned that in the sunlight green plants absorb carbon 

 dioxide and combine it chemically with water in the making of 

 carbohydrate food. During the process oxygen is given off. In 

 the aquarium, the fish and other animals use this oxygen in respi- 

 ration, and throw off carbon dioxide resulting from the chemical 

 combination of the oxygen with the food elements in their bodies, 



Brown Brothers. 



A BALANCED AQUARIUM 



The materials needed for the continued life of both animals and plants 

 are mutually provided. 



thus providing the carbon dioxide needed by the water plants in 

 making food. This process is often referred to as the carbon- 

 oxygen cycle. 



In the aquarium the fish and other animals change a portion 

 of the plant food they eat into the protein substance of their 

 bodies. When this substance is decomposed and excreted in the 



