64 MAINTAINING FISHES 



with an air-lift water pump and usually contain glass 

 wool as a mechanical filter and animal charcoal to 

 absorb dissolved gases. 



A more recent filtering system removes water from 

 below a substratum of sand and gravel. This causes 

 the water to circulate dov^niward and through the sub- 

 stratum. This is, of course, a mechanical filter and 

 cannot be expected to remove dissolved gases. 



The first type of filter also has definite hmitations. 

 In the conventional design the pressure moving the 

 water through the filter is furnished by the column of 

 water above the filtrant. The air-lift pump installed in 

 an aquarium can only Hft water three or four inches. 

 Thus, the column of filtrant must be short and the 

 filtrant must be coarse. When one is dealing with 

 tanks of a hundred or more gallons a small submersible 

 pump may be used to hft the water several feet and 

 provide pressure for filtering through longer columns 

 and finer filtrants. 



It is, of course, also possible to construct a pressure 

 filter which would involve the use of a closed filter 

 with water circulated through it by a submersible 

 type centrifugal pump. 



Synthetic Water 



The mineral content of water varies drastically 

 both in regard to kinds and amounts. Some minerals 

 have a pronounced effect upon the physiology of fishes 

 and upon materials added to the water for experimen- 

 tal purposes. The problem of variation in mineral con- 

 tent has resulted in conflicting reports concerning the 

 effect of toxicants on fishes and on parasitic organ- 

 isms. To avoid this variable there appears to be a 



