Oldest system crashing. Help

[chris taylor Swamp Creek Farms]

Ibc tote system. 4 years old or more channel cats. I added 100 cat fingerlings few months ago. They are dead now and are starting to get to my old fish. I haven't had many problems that I couldn't figure out. Changed the water out twice. All water levels seem to be fine. The fish kinda hang out together and seem to barrel roll when swimming. My friend added 100 from the same company the same day and his are all dead from ick. It sounded like ick but these fish don't have spots or any different colors. I don't know about this one. Maybe I got a bad batch. I haven't lost a fish in a year or more.

[Rick Stillwagon]

If EVERYTHING is ok with your water, then it has to be a pathogen.  Make sure the water is warm, plenty of DO, and salt the water, if you want to try to save the fish.  I would stop circulation through the plant beds, troughs, whatever, and use a biofilter. Treat it like an aquaculture system.  Plenty of water changes, warm water, salt, and hope they recover.  Once the fish are stabilized, gradually go back to your AP system.