I would like to ask your opinion about a desease, which killed my friend's fish.
Tank - 54 litres, water changes 30% every week (tap water witn conditioner and a tittle RO water), parameters: pH 7, kH 3, gH 6 (German degrees), NO2 and NO3 0. A lot of live plants.
In the tank there were 6 betta splendens females, at the age of almost two years (own breeding, hatched in November 2008).
In July this year something strange shown up on the fish side. At the beginning it was small and looked like this:
However, in time it grown
Finally the fish died on 7th October. Until the last 2 days, it behaved normally, as if completely healthy.
Neither this fish nor any other in the tank had been ill ever before, also the parents did not show any illness symptoms. The parents were not related.
Has anyone any idea what it might be?
Thank you in advance?