In the aquarium with a circulating system, nitrification (ammonium and nitrite oxidation) is one of the most important reactions which are found in the filter-sand bed. This paper deals with the nitrifying activity of the filter-sand used in the aquarium in which fish has been cultivated for a long period and then the breeding water has reached to an equilibrium in its quality.
The results obtained are as follows.
1. A number of nitrifying bacteria, as well as different biochemical types of heterotrophs, is found in the upper layer of the filter-sand bed, and the lower the layer, the more decrea-sed in the bacterial counts. By which, the nitrifying activity is the largest in the upper layer of the bed.
2. The nitrifying activity of the sand decreases gradually when the sand is washed with sea water, however, a somewhat large activity still lies in the sand even after washed.
3. The nitrifying activity of the sand is the largest at 30°-35°C and at pH ca. 9.0, though somewhat different with sea water or fresh water aquarium.
4. The filter-sand of sea water aquarium shows the largest nitrifying activity in the solution of natural sea water or saline with the same salt concentration. Contrary to this, the higher the concentration of sea water or sodium chloride, the lower the activity in the case of the sand of fresh water aquarium. Nitrification of the former sand is completely inhibited in fresh water, and also that of the latter in sea water or in saline of the same salt concentration.
5. The nitrifying activity of the sand decreases under low oxygen tension. However, the nitrification occurs in a fair degree even under extremely low oxygen tension. Effect of aeration on the nitrification of sand of a sea water aquarium is different from that of fresh water one, namely, the activity of the former sand decreases by aeration but it is not so in the case of the latter.
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