6o MAINTAINING FISHES 



weight in gaseous chlorine. The most available form 

 of dissolved chlorine is common household bleach. 

 The choice of form depends upon the size tanks to be 

 sterilized and the amount of sterilizing to be done. 

 The gaseous form is the cheapest but is less available, 

 harder to handle, and dangerous. Calcium hypo- 

 chlorite is fairly cheap and convenient to use. Chlorox, 

 Hilex, and other household bleaches are convenient 

 to use but are more expensive. 



Chlorine solutions may be used in two ways. For 

 floors, benches, large tanks, etc., the surface may be 

 rinsed or mopped several times with a 200 p.p.m. or 

 stronger solution of chlorine. Small tanks may be filled 

 with the chlorine solution, and small objects may be 

 submerged for thirty minutes. In some cases it is well 

 to maintain a crock of chlorine and a crock of neutral- 

 izing solution ( discussed below ) to permit dipping of 

 nets and other equipment which might transfer dis- 

 ease from tank to tank. 



Chlorine is effectively neutralized by sodium thio- 

 sulfate (photographic "hypo"). Following steriliza- 

 tion of tanks and other equipment with chlorine, they 

 should be rinsed with a 100 p.p.m. solution of this 

 compound. The tanks should then be rinsed with wa- 

 ter after which they may be used. 



In cases where the use of chlorine is not desirable, 

 Roccal, a germicidal agent sold by Winthrop-Sterns, 

 may be used. 



Circulation and Aeration of Water 



Oxygen is only slightly soluble in water; even at 

 saturation the available reserve is limited, and oxygen 

 must be continually replenished. In addition the re- 



