VARIABLES OF AQUARIUM WATER 67 



submerged, green plants, especially algae. Dissolved 

 oxygen is depleted by respiration by plants and ani- 

 mals. If some pollution is present, such as an accumu- 

 lation of uneaten food, bacteria may rapidly deplete 

 the available oxygen present in water. 



Water's oxygen-holding capacity varies inversely 

 with the temperature; i.e., the higher the temperature 

 the less the amount of oxygen that tlie water will hold. 

 The following relationships illustrate this: At tem- 

 peratures of 50°, 60°, 70° and 80° F. the oxygen re- 

 quired for saturation is 11, 10, 9, and 8 p. p.m. respec- 

 tively. 



An oxygen concentration of 3 to 5 p.p.m. is ade- 

 quate for warmwater fishes and many can survive at 

 lower concentrations. 



TAKING WATER SAMPLES: Water samples 

 used for oxygen determination must not be exposed to 

 air during sampling. To obtain a nonaerated sample 

 from an aquarium the simplest procedure is to use a 

 small rubber tube to siphon water into a 250 ml., glass- 

 stopper type bottle. One end of the siphon tube 

 should be placed in the aquarium and the other end 

 near the bottom of the bottle and the water permitted 

 to overflow three times the volume of the sampling 

 bottle. The sample should be analyzed immediately 

 after it is taken. 



REAGENTS REQUIRED: (J) Manganous sulfate 

 solution (dissolve 480 g. MnS04-4H2O in 100 ml. of 

 distilled water and dilute to 1 L. ). (2) Alkaline-iodide 

 sodium azide reagent (add 700 g. KOH, 150 g. KI to 

 950 ml. of distilled water; permit to cool; Add 10 g. 



