Question: asking about bacterial diseases in fresh water fish
Answer: advice him to apply 
(1) apply potash(kmno4) to the infected fish pond at the rate of 4 mg/lit. of water. (or)
(2) give short bath treatment to the infected fish in common salt (nacl) solution at a concentration of 2.5-3% for 5 –10 minutes. treated fish should be released to the pond immediately after the treatment. (or)
(3) apply cleaner solution to the infected fish pond at the rate of 50- 100ml/ bigha water spread area. if draining is possible then lime at the rate of 200- 250 kg/ ha area can be applied to the drained pond bottom and the allowed to dry for 2 weeks to eradicate the parasites completely. after eradication of the parasite restocking is in the pond is done