4 MAINTAINING FISHES 



The presence of fishes increases the dissolved salt 

 content of water and alters the composition of its ionic 

 system. The effect of this change on fishes is not 

 known. Short of pollution, fishes alter water in such a 

 manner as results in iacreased growth of other fishes 

 subsequently occupying it. This phenonemon is 

 known as conditioning. 



Fishes can be crowded to the extent that both re- 

 production and growth are inhibited. Exactly what 

 these inhibiting substances are is not known. The 

 situation may be nothing more than a combination of 

 waste materials concentrated enough to be generally 

 toxic to fishes and thus interfere with all normal 

 functions. 



Natural processes eliminate or inactivate some 

 waste materials. Green plants, including algae, take 

 up large quantities of carbon dioxide and release 

 oxygen. Bacteria break down fecal and unused food 

 materials. Snails and other invertebrates utilize ex- 

 cess food materials. Chemical reactions cause the 

 breakdown and inactivation of many wastes. Still 

 other wastes are lost as gases at the water's surface. 

 But none of the natural processes are entirely ade- 

 quate when fishes are crowded beyond a certain 

 point. We do not presently know the range of density 

 values which constitute the upper limit that can be 

 accommodated by natural processes. In terms of 

 grams of fish per liter of water it is probably less than 

 one gram per hter in the absence of an algal bloom 

 or other plant growth. Higher densities appear pos- 

 sible when plant growth is prevalent. The relation- 

 ship wiU vary with species of fish and a number of 

 other considerations. 



Among tropical fish hobbiests the rule of thumb for 



