From your earlier description of the two ponds: your system filters
water from the bottom pond and dumps that back into the top pond.
Effectively, the water that supplies the bottom pond is unfiltered (it
includes wastes from fish in the top pond. You state that ammonia is
0. Is this true of both ponds? And, have you checked the ammonia
(and other parameters) after the fish began suffering? If ammonia
levels exceed 0.5 mg/L, fish may clamp fins and are much more
susceptile to disease and other environmental stresses.


(Gallandro) wrote:

The only thing I can come up with is maybe it is Costia ? although
there is no redness or gulping for air. but they do go off there
food, retreat to the bottom of the pond before comming up to die.