70 MAINTAINING FISEOES 



hence more important. Thus, when the amount of il- 

 lumination is excessive, algal growth may become 

 pronounced. During hours of illumination all carbon 

 dioxide present may be utilized by algal photosyn- 

 thesis, but during periods of darkness photosynthesis 

 ceases, respiration continues and carbon dioxide con- 

 centrations go up. Another common source of excess 

 carbon dioxide is from organic pollution such as re- 

 sults from over-feeding. In this case high carbon diox- 

 ide is due to bacterial respiration. 



In most cases carbon dioxide does not constitute a 

 problem in holding fishes in aquaria. At a concentra- 

 tion of 150 p.p.m. it will anaesthetize fishes and at 

 higher concentrations it is usually fatal, but these high 

 concentrations do not commonly occur. Carbon 

 dioxide is quite soluble, but low values (less than 20 

 p.p.m. ) are more characteristic of aquarium water. 



TAKING WATER SAMPLE : Siphon 100-ml. water 

 sample into a graduate. With a minimum of agitation 

 transfer this sample to a beaker. 



BEAGENTS REQUIRED: (I) Sodium hydroxidc 

 solution N/44 (dissolve 0.909 g. of NaOH in 500 ml. 

 of distilled water and dilute to 1 L.; standardize 

 against any standardized acid solution). (2) Phenol- 

 phthalein indicator (0.5% solution in 50% alcohol). 



TITRATION procedure: To a 100-ml. sample 

 of water add 10 drops phenolphthalein indicator and 

 then titrate with the sodium hydroxide solution to a 

 faint, permanent pink. The milliliters of sodium 

 hydroxide solution used multiplied by 10 and by the 



