Fish rearing system

Claim 1

A fish rearing system in which water can be recycled, the system comprising the following components in combination: a. a fish tank containing fish and having a recycle water inlet and an outlet; b. primary filtration means having an inlet communicating with the outlet of the fish tank, a sludge outlet and a second outlet for recycling water, the primary filtration means being able to remove a substantial proportion of particulate matter from the water coming from the fish tank with low retention time of the water in the primary filtration means and without blinding of the filtration means; c. a surge tank having an inlet communicating with the second outlet of the primary filtration means, a first outlet for waste and a second outlet; d. a pump communicating with the second outlet of the surge tank and having an outlet; e. a biological filter for oxidizing toxic ammonia nitrogen into the non-toxic nitrate form, said filter communicating with the outlet of the pump, the filter comprising a container having a false floor an inlet for recycled water above the false floor and an outlet for backwashings above the false floor, an outlet for recycled water, an inlet for backwash air and an inlet for backwash water below the false floor; f. a particulate filter medium having particles of a size in the range 0.5 to 10 mm. positioned on the false floor; g. water backwash means whereby backwash water can be forced through the inlet for backwash water in the biological filter and through the particulate medium of the filter to clean the medium when required; h. air scour means whereby air can be forced through the inlet for backwash air in the biological filter and through the particulate medium of the filter to lift and expand the medium and agitate the medium vigorously to permit adequate backwashing of the medium by the backwash water; i. a stilling box communicating with the outlet for recycled water of the biological filter, the stilling box having an outlet; j. a sump communicating with the outlet of the stilling box and having an outlet; k. a recycle pump communicating with the outlet of the sump; l. an ozone contact unit in which a predetermined amount of recycling water is contacted with ozone in an amount sufficient to lower the total nitrite content of the water to a level acceptable to fish but which amount does not leave free ozone in the water re-entering the rearing tank; m. an aeration unit positioned between the filter outlet for recycled water and the recycled water inlet of the fish tank whereby water in the system can be aerated and stripped of carbon dioxide; n. a fresh water inlet in the system; and o. conduit means whereby water can be fed throughout the system.