Too low pH. Can be caused by unsuitable water supply, reductive processes which take place in the tank/pond, pollution of the environment etc.
Can be remedied if the cause is determined and can be treated. In tanks where there is control of the environment treatment is possible. In ponds/cages it is much harder. Outlook is excellent if pH can be raised.
Dr. Bullock, Graham L.
US Fish & Wildlife Service
Nat. Fisheries Res. C.
'Pete' Bullock, one of the founding luminaries of Fish Pathology