THE AQUARIUM 73 



containing some plants, some vegetarian animals such as pond- 

 snails, and a few other forms of animal life, which shall require 

 little or no attention beyond the adding of fresh water as the 

 old evaporates in the warm room. One would like to drive home 

 the fact that ultimately the whole round world swings round 

 the green plant, that all flesh is grass, or, as it has been put by a 

 modern, all fish is diatom. Without the green plant life would 

 come to a standstill. It alone has the power of drawing fresh 

 energy from sunlight, and it is upon that energy that all living 

 things ultimately depend. 



In practice, however, the state of perfect balance is excessively 

 difficult to preserve within the limits of a practicable aquarium, 

 and we must consider the difficulties which arise. To begin 

 with the plants what difficulties confront the aquarium keeper 

 in regard to them ? We may say first that though they may suffer 

 from lack of light and warmth, they never in captivity suffer 

 from a lack of carbonic acid. Nor, in an ordinary aquarium, 

 are they at all likely to suffer from a lack of nitrates and other 

 necessary salts in the water. On the other hand, they are quite 

 frequently poisoned by an excess of food substances, by the 

 presence in the water of products of decomposition far in excess 

 of the amount which they can absorb, or even tolerate. As to 

 the animals, their untimely death in the aquarium may in the 

 general case be ascribed to one of two causes either to the 

 presence of an excess of carbonic acid, combined with a paucity 

 of oxygen, or to the presence in the water of products of decom- 

 position to a poisonous extent. Of these the first is by far the 

 most frequent. Take a little fish from a swift -running stream 

 and put it into your collecting bottle. In a very few minutes, 

 probably, especially if the bottle contains other animals, the 

 fish will begin to gasp, it will rise to the surface in the attempt 

 to get air there, it will show by its change of colour, and the 

 paling of its gills, that it is suffering from oxygen starvation. 

 Waste products or products of decomposition do not accumulate 

 in lethal doses with such rapidity the little fish suffers because 

 there is too much carbonic acid and not enough oxygen. If the 

 bottle is deep in proportion to its circumference even the addi- 

 tion of water-weed may not save the little creature, for the weed 



