noticed a lump on the tail of one of my koi a few years ago, have treated with various over the counter medicines mainly anti bacterial and anti funcus one of which turned the water green for about two weeks, that actually shrunk the lump and i was hopefull of a cure only for it to regrow bigger and darker in colour. it does not seem bothered eats well and does not affect swimming although it's growth is slow compared to other fish, do have over 30 fish in pond no others have been affected.
any advice welcome thanks.