A BALANCED AQUARIUM 



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have strong stems, and the leaves are usually divided and present 

 little resistance to the water. The roots are small and are not of 

 much use as an anchor. The fish are obviously adapted for life 

 in the water, as we have already seen. Even the snails have 

 adaptations for their life in the water. 



We have learned that green plants, in favorable conditions of 

 sunlight, heat, moisture, and with a supply of raw food materials, 

 give off oxygen as a by-product while manufacturing food in their 

 green cells. We know 

 the necessary raw ma- 

 terials for carbohydrate 

 manufacture are carbon 

 dioxide and water, while 

 nitrogenous material is 

 necessary for the making 

 of proteins within the 

 plant. In previous ex- 

 periments we have 

 proved that carbon di- 

 oxide is given off by 

 living things when oxida- 

 tion occurs in the body. The crawling snails and the swimming 

 fish give off carbon dioxide which is dissolved in the water ; the 

 plants themselves, at all times, oxidize food within their bodies, 

 and so must pass off some carbon dioxide. The green plants in 

 the daytime use up the carbon dioxide obtained from the various 

 sources and, with the water which they take in, manufacture 

 carbohydrates. While this process is going on, oxygen is given off 

 to the water of the aquarium, and is used by the animals there. 



The plants are continually growing, but the snails and fish 

 eat parts of the plants. Thus the plant life gives food to the 

 animals within the aquarium. The animals give off certain 

 nitrogenous wastes. These materials, with other nitrogenous 

 matter from dead animals and parts of the plants, form part of 

 the raw material used for protein manufacture in the plant. This 

 nitrogenous matter is prepared for use by several different kinds 

 of bacteria which break down the dead bodies and change the 



Why will it not be necessary to change the water in this 

 aquarium ? 



