Old system is crashing

[chris taylor Swamp Creek Farms]

I have had this IBC system for about 4 years now,  never really had a problem i couldn't figure out.   I bought 100 channel cat fingerling a few months back and they started to die about two weeks ago.  Now they are all dead and also they are starting to get to my old catfish.  They kind of barrel roll and pack up together in schools.  The water levels seem fine.  I changed the water out twice,  i have not had to change it out since i first started.  The plants seem to grow fine,  no stress or anything showing.  My friend also bought 100 cats from the same place and his all died from "ich"  or that is what it sounded like.  Possibly they had a bad batch and those gave mine something? His fish did not last a few weeks, where mine lasted 5 months.   Everyday I am losing four or five fish.   I don't really care about the fingerlings cause i know some would die off,  but the big ones ive grown forever are my favorites.  I put about 15 in my main pond to save them.

They don't have any spot on them to make me think it is ich.   The sporadic swimming I wonder about.  

i think the levels are around 0 ammonia, 8 PH, 0 nitrites.   some where close to those.