MINIATURE CIRCULATING SYSTEMS FOR SMALL LABORATORY AQUARIUMS 51 



purposes. It consists in titrating a sample 

 with N/100 hydrochloric acid to which 

 brom-cresol purple has been added as an 

 indicator. After the purple color has 

 vanished, the sample is repeatedly boiled 

 and further titrated until the purple 

 color no longer reappears on heating. If 

 the sample consists of 100 cc. to which five 

 drops of indicator have been added, the 

 final burette reading in cc. multiplied by 

 0.1 gives the bound CO2 or bicarbonate 

 in millimols/liter. This method is not to 

 be generally recommended for accurate 

 work but is sufficient as a comparative 

 measure of how far and how fast the aging 

 water is departing from its original value. 

 With this information, corrective meas- 

 ures may be taken. The specific gravity 

 is nearly taken care of by automatic means 

 involving the use of the float valve al- 

 ready discussed. Under normal opera- 

 tions distilled water is used to make up for 

 the evaporation of sea water, which of 

 course tends to increase its density thereby. 

 This has been satisfactorily supplied 

 from a 5-gallon carboy on a shelf higher 

 than the float valve. The operation of 

 the float valve holds the amount of water 

 in the system at a constant volume, which 

 means also that the dissolved salts will 

 remain at a constant amount. If it is 

 desired to increase the density of the 

 water, instead of using distilled water as 

 an additive, sea water may be used until 

 the specific gravity has reached the de- 

 sired level. If it is desirable to reduce 

 the salinity, water may be withdrawn 

 from the system while distilled water is 

 used in the float- valve supply. This may 

 be conveniently accomplished by means of 

 a siphon with a small hose clamp so that 

 the flow is restricted to a drip slow enough 

 to permit the float valve to follow. Al- 

 though distilled water was customarily 

 used, in its absence tap water was used 

 with no detectable effect on the fishes or 

 the system. 



If the pH falls to lower values it may 

 mean that there is an increase in the 

 amount of free CO2 present. This could 

 indicate too many organisms for the vol- 

 ume and temperature of the water or 

 too much decomposition for the antacid 

 components of the system to dispose of 

 rapidly. The calcium carbonate in the 

 sand should react with the acids formed, 

 and unless there is overcrowding this type 

 of decreasing alkalinity usually does not 

 present a problem. If the placing of fresh 

 activated bone charcoal in the filter results 

 in an abrupt increase in the pH, it is al- 

 most certain that there is too much free 

 CO2 present. The use of charcoal re- 

 newed at short intervals will bring the 

 CO2 content down, but the charcoal rap- 

 idly becomes saturated and cannot be 

 thought of as a regular part of the regu- 

 latory process. An increase in the num- 

 ber of aeration stones or amount of air 

 they pass, while much slower in its effects, 

 is a much more satisfactory way to insure 

 against the accumulation of CO2. 



If, on a falling pH, none of the pro- 

 cedures above mentioned increase the pH 

 significantly, the titration reading should 

 be carefully checked and it too should 

 show a decrease. This would indicate a 

 lowering of the bound CO2 which does not 

 normally occur in an unoverloaded system 

 in the presence of calcareous sand. If it 

 does, however, more sand may be added, or 

 sodium bicarbonate may be dissolved and 

 administered with the distilled water 

 through the float valve. Since the sand 

 alone tends in a long-term sense to dispro- 

 portionately increase the Ca in solution 

 as compared with the Na, the occasional 

 use of sodium bicarbonate, which tends to 

 do the reverse, aids in keeping these two 

 quantities in more nearly normal propor- 

 tions. See Breder and Smith (1932). 



The described procedures may seem to 

 be somewhat complicated, but they are, in 

 fact, not much more complex than those 



