aren't there always some? My striped kuhli Sassy has never gained weight like all the others. She quit wasting away, but isn't actually improving. I have treated the tank several times now with different meds for internal parasites, not sure what else it could be.
Two of my peppered cories don't look well. I wondered if the smallest one was a runt or unwell, because it didn't grow like the others- four are now the same size, one has remained small. Perhaps it will always be smaller. But now one of the fullsize cories appears listless and weak- the current pushes it around and often I find it motionless among some plant stems, twice I have found it limp drifted against the pothos roots or underside of the fry box. I haven't treated with anything because I don't know what's wrong with it. I thought the frequent small wc I'm doing for the fry would help it- but it seems to be going downhill.
I also worried that something I did recently has harmed it. I added more pothos stems across the back of the tank (stuck through a strip of plastic that has regular holes, from one of the kids' building kits). And I'm using wood stain on the tank stand in progress- I'm working in the basement, running fans and keeping the doors closed between, but still I can smell the fumes in the house. But none of the other fishes seem affected, especially the fry I would think is more sensitive.