To provide a method and an apparatus for cleaning rearing water of an aquatic life, dispensing with the use of chemicals and a complicate operation management, effective for suppressing the increase of the turbidity and color of the water, eliminating proteins, total nitrogen, ammoniacal nitrogen and nitrite-nitrogen from rearing water and automatically keeping the water to neutral state by preventing the lowering of the pH of the water to be treated.

The rearing water cleaning method comprises a filtering step to use sand as the filtering material in the cleaning of rearing water 2, a pH adjusting step to use oyster shell and a foam separation step. The foam separation step performs foam separation treatment of the rearing water 2 by supplying a gas to the rearing water 2 in a separation tank 7, the gas is supplied to the separation tank 7 in the form of fine bubbles, the fine bubbles are formed by generating bubbles by a means for exclusively generating bubbles and then dividing the bubbles generated by a bubble-shearing means and/or formed by the bubble-shearing means itself, and the fine bubbles are used in the step to perform the foam separation treatment of the rearing water.