Tolerance to Formalin by a Fluidized‐Bed Biofilter and Rainbow Trout Oncorhynchus mykiss in a Recirculating Culture System

Formalin tolerances of a fluidized‐bed sand biofilter and rainbow trout Oncorhynchus mykiss were tested in a semi‐closed freshwater recirculating culture system. Progressively higher formalin levels were tested until fish mortality (±1.9%) occurred. A 1‐h exposure to 167 ppm formalin, followed by flushing, was safe for trout (no mortality) at 15.0 C but not at 16.5 C. A 110‐ppm indefinite treatment (no flushing) was safe at 17.3 C, but 120 ppm at 17.3 C and 100 ppm at 17.8 C were not. Biofilter nitrification was not impaired by 1‐h formdin treatments up to 167 ppm at 16.5 C, and not, usually, by indefinite treatments up to 120 ppm at 17.3 C. However, a find indefinite treatment of 70 ppm was followed by abnormally high nitrite levels for 9 d. Formaldehyde remained detectable in the system for 11 h during indefinite treatment at 120 ppm formalin. Tests on the system's Co2 stripper did not indicate that it removed formaldehyde.