Short-term toxicity of various pharmacological agents on the in vitro nitrification process in a simple closed aquatic system

During the treatment of fish diseases, drugs which inhibit the nitrificatio n process can cause acute ammonia toxicity. The same phenomenon can occur w hen fish are put into a tank without active cultures of nitrifying bacteria . The purpose of this study was to quantify the inhibitory effects of 15 ph armacological agents, which are often used as therapeutic agents in ichthyo pathology, on ammonia removal and nitrate production in a simple closed aqu atic system. The experiments were conducted in polyethylene bags containing activated biofilters and synthetic water solutions, held in a water bath. Ammonia was added to initiate the nitrification process, and graded concent rations of various pharmacological agents were added. The effects of the ph armacological agents on in vitro nitrification were assessed by monitoring ammonia and nitrate concentrations compared to controls with no added agent s, for 24 hours. Graded concentrations of ampicillin (Albipen(R)), chlorami ne T, enrofloxacin (Baytril(R)), erythromycin, levamisole, methylene blue a nd polymyxin B induced dose-dependent inhibitions of ammonia removal and ni trate production. The corresponding linear regression curves showed high co rrelation coefficients and were highly significant (p < 0.05). The addition of chloramphenicol, copper (II) sulphate, kanamycin disulphate, malachite green, neomycin sulphate, potassium penicillin G, tetracycline and a mixtur e of trimethoprim and sulphadoxin (Duoprim(TM)) had no significant effects on the nitrification process. A significant dose-related inhibition of nitr ate production, but not of ammonia oxidation, was observed with enrofloxaci n. The significant correlation (r = 0.940; p < 0.001) between the degrees o f inhibition of ammonia oxidation and nitrate production for the various in hibitory pharmacological agents has also been calculated, with a view to va lidating this method. The data presented suggest that separate tank facilit ies for hospitalisation or quarantine are necessary when treating diseased fish with ampicillin, enrofloxacin, chloramine T, erythromycin, levamisole, methylene blue or polymyxin B, in order to avoid ammonia poisoning.