Toxicity of formalin for fingerlings of Cyprinus carpio var. koi and in vitro efficacy against Dactylogyrus minutus Kulwièc, 1927 (Monogenea: Dactylogyridae).

The toxicity of formalin on Cyprinus carpio var. koi and its anti-parasite effects against Dactylogyrus minutus (Monogenea) in in vitro tests is analyzed. Specimens of D. minutus were submitted to eight concentrations of formalin: 50, 75, 100, 125, 150, 175, 200, 250 mg L-1, in triplicate. Concentrations of formalin 100, 150 and 200 mg L-1 were then tested to determine the median lethal concentration of 50% of the fish per immersion bath. Fish behavior was also observed during the first 6 h of exposure. The 200 mg L-1 concentration was the most rapid efficacy for D. minutus, killing all parasites in 16 min. All parasites were killed in 47 min at concentration 100 mg L-1. Concentration 200 mg L-1 was the most lethal for fish in less than 24 h exposure, with 24 h LC50 at 135.44 (119.78-153.14) mg L-1. The therapeutic index was 2.05-30 min and 1.15-16 min. A short bath (1 h) is recommended in koi carp with a minimum concentration of 75 mg L-1 of formalin, not exceeding 100 mg L-1 for treatment against D. minutus.