Effects on Nitrifying Bacteria

Chemicals used to treat fish diseases and parasites can be toxic to nitrifying bacteria at therapeutic levels for fish. Research has shown that there are some differences in the inhibitory effects of formalin, malachite green, methylene blue, copper sulphate, and potassium permanganate on the biofilter bacteria. Different studies show different effects.

Formalin used in one study, at 25 mg/L had no effect, whereas another study showed reduction of biofilter bacterial activity by 27% when used at 15 mg/L. As a rule of thumb, most aquaculturists do not consider the use of formalin at 15–25 mg/ L to have a major impact on the biofilter. However, when testing for ammonia levels, formalin will react with Nessler’s Reagent (a component of most ammonia test kits) and can give a falsely elevated ammonia reading. In systems treated with formalin, the salicylate reagent test for ammonia is recommended because it does not react with aldehydes (e.g., formaldehyde found in formalin).

Malachite green has been shown to have no effect on the biofilter at 0.1 mg/L, combined with or without formalin at 25 mg/L. Copper sulphate at 1 and 5 mg/L likewise had no effect on biofiltration.

By contrast, potassium permanganate experiments have been mixed. In one study, a 4 mg/L dosage resulted in no inhibition of the biofilter, whereas in another study, a 1 mg/L dosage resulted in an 86% inhibition. The actual impact on an individual system will most likely depend upon many factors, such as chemical concentration, length of time in treatment, organic load, pH, temperature, alkalinity, filtration, oxygen levels, and stocking density; and, although this will most likely be true for most chemicals, this may better explain the differences in effect by potassium permanganate.

Antibiotics should never be used in aquarium systems because of severe detrimental effects on the bacteria within the biofilter. If a population of fish in a recirculating aquarium system must undergo a specific antibiotic treatment, the biological filtration system should be shut off during treatment. After treatment, a large water changes for the treated aquarium is recommended.