NOVELTY - A high-density adsorbent for bacteria is one in which a large number of microorganisms are fixed at a high density and the microorganisms amongst which are nitrifying or sulfating bacteria that can be cultured at high density.    USE - The adsorbents for bacteria are applicable in closed circulatory systems for fish farming, fish fry culture and in seafood production, and are useful in fishery.    ADVANTAGE - On such adsorbents, microbial cells of nitrifying bacteria and denitrifying bacteria are fixed at high density as food to help e.g. fish to grow.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a similar adsorbent in which the microorganisms are nitrifying bacteria that can be acclimatized and accumulated from active sludges like water and urine sludges by culturing in a medium biased toward the acidic side at pH 7-9, at a dissolved oxygen concentration of not less than 2 mg/l, at 20-40 degrees C over a definite period of time, with acclimatization in a liquid containing NH4-N and a mixture of sodium carbonate and sodium bicarbonate as culture promoter for maintenance to produce the nitrifying bacteria at high density from such active sludges;    (2) another similar adsorbent in which denitrifying bacteria are accumulated at high density from the active sludges, at temperature of 10-40 degrees C, external carbon source of ROH (where R = methyl and/or ethyl) and with a liquid containing NO3-N for denitrifying acclimatization;    (3) a fish fry culture system comprising at least a water tank for feeding and raising, a pH controller, a supply unit for highly dissolved oxygen, and a nitrification tank equipped with the adsorbent inside for producing bacteria at high density, with the feeding and raising unit capable of discharging waste water through the other parts and tanks before purification for recyclization in a closed circulatory system for culturing; and    (4) a closed circulatory system for fish farming and fish fry culture similarly constructed in which nitrifying and/or denitrifying reactors are installed for the required reaction in acclimatizing the respective bacteria at high density on the adsorbent, with a mixture of sodium carbonate and sodium bicarbonate to control pH and act as carbon source for nitrifying bacteria and regulation of ammonia concentration not necessary by increasing the amount of adsorbent.    DESCRIPTION OF DRAWING(S) - Diagram of a nitrifying acclimatization apparatus for sludges, equipped with thermostat, heater, nitrifying tank, pH-stat, pump, and supplied with air and sodium carbonate and sodium bicarbonate mixture. (Drawing includes non-English language text).