10 MAINTAINING FISHES 



are produced by decay and as waste products from 

 fishes. These can be prevented from accumulating 

 by aeration and charcoal filtration. 



The accumulation of soluble salts both by evapo- 

 ration and from excretion is best avoided by fre- 

 quently replacing a portion of the water of each tank 

 or by continuously flushing the tanks. 



The repression of reproduction appears to become 

 evident when populations are unusually dense. Thia- 

 ning a population is an obvious approach. Continuous 

 flushing may also be a solution. 



Growth repression may function at a much lower 

 level of population density than reproduction repres- 

 sion. The author does not feel that enough is known 

 about this phenomenon to make possible recommen- 

 dations for avoiding it. 



Classification of Fish-holding Facilities 



In order to increase the grams of fish per liter of 

 water, the following artificial aids are commonly used: 

 frequent water changes, aeration, filtering and flush- 

 ing. Whether or not artificial aids are used and to 

 what extent will considerably affect the physico- 

 chemical conditions of the aquarium water. The var- 

 iation in artificial aids in maintaining desirable 

 aquarium conditions must be recognized in deciding 

 what type of set-up is best suited to a particular need. 

 We, thus, may consider the different arrangements 

 one might set up and the use to which they are best 

 suited. 



The first and simplest arrangement involves no re- 

 placement or artificial aids for neutralizing wastes and 

 replenishing oxygen. The weight of fish to volume of 



