WATERS AND HOLDING PROCEDURES 11 



water must be low enough to permit natural processes 

 to maintain the water in a condition satisfactory for 

 fishes. Higher aquatic plants are often added to this 

 set-up. They increase the available oxygen and de- 

 crease the carbon dioxide, at least when they are ex- 

 posed to light, and probably have the over-all effect 

 of making it possible to hold more fish per volume of 

 water. This arrangement most closely resembles natu- 

 ral pond conditions and is the best choice of arrange- 

 ments when one seeks a good choice for display 

 tanks. It requires less care, may be made to look quite 

 natural and does not require a water supply or drain 

 service. 



The second arrangement includes a partial change 

 of water daily and continuous aeration. This type of 

 arrangement permits holding a considerable weight of 

 fish in a limited space and without much special equip- 

 ment. It is of value for testing toxicity of chemicals 

 and for the study of rapidly developing diseases such 

 as those caused by bacteria and some protozoans. 



In the third basic type set-up the water is circulated 

 and filtered. This type of holding facility is of special 

 value where one wishes to maintain dense populations 

 of fish over an extended period of time and there is a 

 scarcity of good water of the correct temperature. 



The fourth arrangement involves continuous re- 

 placement of the water by flushing. The amount of 

 flushing may vary considerably. Holding tanks in gold- 

 fish hatcheries may have a flush rate high enough to 

 give the water a change every twenty to thirty minutes, 

 but for aquaria where the fish are much less crowded, 

 a flush rate giving a complete change in twenty-four 

 hours is suflBcient to maintain clean water. Of course. 



