Investigations into the effects of concentration and duration of exposure to formalin and malachite green on the survival of the larvae and juveniles of the common carp Cyprinus carpio L. and the sharptooth catfish Clarias gariepinus (Burchell).

Prophylactic dip treatments using formalin and malachite green were applied to 4-day old larvae and 12- and 20-day old juveniles of the European common carp, Cyprinus carpio and the African sharptooth catfish, Clarias gariepinus. Treatments consisted of 100 mg/l malachite green for exposure periods of 10, 30 or 90 s and 200 mg/l formalin, administered for 30, 60 or 90 min. Larvae and juveniles of C. gariepinus could be treated with 100 mg/l malachite green for 10 s, or with 200 mg/l formalin for 30 min, with minimum mortalities. Both chemicals affected the survival of the C. gariepinus juveniles, especially the 90 min exposure to formalin. Juveniles of both species were extremely sensitive to 100 mg/l malachite green concentrations.