Lost corals after clean the sump

[LaloJ]

I don't see how that answers my question. You said that high nitrates were hurting your fish and when you did a water change, the fish recovered. But it could've been something else in the water that was hurting the fish that was removed when you did the water change.

That may be true, I can't think of anything else inside the tank affecting the fish, it's just a FOWLR, I didn't add anything, there are no invertebrates or anemones, there are no corals, there are no reactors, I didn't use any "miracle product" or anything similar.