Formalin is a common parasiticide used to treat many fish species. The effect of formalin on the biofilter, however, is not well understood and current data are contradictory as to its impact on the nitrification process of bacteria.1,2,3 The biofilter of the infected system is often left untreated as a result, which leaves an ample environment for parasites to thrive. Therefore, understanding formalin's effects on the biofilter is important to ensure the complete eradication of the parasite from the fish's habitat and prevent re-infestation.
A model was established to assess formalin's effects on nitrification in an artificial saltwater environment. Six 15 L recirculating systems, consisting of three control and three experimental units, contained equal amounts of biomedia and were fed daily with a known amount of ammonium chloride. The experimental units were treated with a common protocol for therapeutic formalin treatment, using the highest suggested concentration of 250 mg/L of 37% formaldehyde solution (Formalin-F, Natchez Animal Supply Co., Natchez, MS) given every third day for a total of six, one-hour formalin baths. The biofilter's efficiency was monitored two to three times daily, measuring both the ammonia and nitrite levels prior, during, and post treatment to calculate the reduction rates of the biofilter. The data showed that the formalin treatment at the highest concentration (250 mg/L, given every third day) did not differ from controls in its effects on the biofilter's nitrification. Therefore, this study concluded that the biofilter could be exposed to the same treatment as the fish thus resulting in a more effective treatment to prevent re-infestation.
1. Borrows R, BD Combs. 1968. Controlled environments for salmon propagation. The Progressive Fish-Culturist 30:123-126.
2. Collins MT, JB Gratzek, DL Dawe, TG Nemetz. 1975. Effects of parasiticides on nitrification. Journal of Fisheries Research Board of Canada 32: 2033-2037.
3. Keck N, G Blanc. 2002. Effects of formalin chemotherapeutic treatments on biofilter efficiency in a marine recirculating fish farming system. Aquatic Living Resources 15: 361-370.