6 MAINTAINING FISHES 



been utilized by fishes but has been permitted to 

 stand long enough to develop a population of micro- 

 scopic and semimicroscopic plants and animals. Sun- 

 light and the addition of a small amount of organic 

 matter result in more rapid and complete aging. In 

 addition to involving the development of microscopic 

 life, aging also permits the loss of excess quantities 

 of gases and the precipitation of toxic minerals 

 sometimes present in new water, especially water 

 from wells and springs. 



Conditioned water is water which has supported 

 fish life. It has been demonstrated that at least under 

 some conditions fish grow better in conditioned water. 

 Most aquarists already recognize the importance of 

 this phenomenon for it is a common practice to add 

 at least some "old water'* to a newly estabhshed 

 aquarium. It is not yet known what is involved in 

 conditioning. Perhaps the best theory to date has 

 been offered by Breder (1931). He suggests that 

 water previously occupied by fishes contains a 

 bacteriophage that controls the growth of undesirable 

 bacteria. Other possible explanations of conditioning 

 might include changes in the ionic system of the 

 water brought about by exchanges of salts between 

 the fishes and the water, or we might say that in the 

 presence of fishes the ionic system of the water is 

 adjusted to more nearly suit that of the fishes' re- 

 quirements. Also relating to the ionic system is the 

 possibility that toxic ions are absorbed by the first 

 fishes and thus leave the water more favorable for 

 the subsequent inhabitants. 



Under the heading of "Polluted Water" it is neces- 

 sary to recognize a number of subdivisions. It will be 



