something went wrong

with my water change- I was pleased with this morning's work, until I walked by the tank a little later. Several of my black kuhli loaches are lying around motionless- I think they just have tummyaches from overeating last night- when I nudge the aquarium tongs near them, they dart away. But my lonely oto died. He was hanging out near the surface right after the water change, then I found him unmoving on the bottom. Not sure why- was the difference of a 50% wc too much for him? I did not forgot the dechlor, I matched the temp pretty close. I did rinse the prefilter sponge and that always kicks quite a bit of mulm into the water column, but it cleared within twenty minutes and no one else seemed ill. One of the female cherry barbs has been occasionally flashing- I feared ich at first but no one has shown white spots, and she's the only one doing this. It happens after I dose ferts, and once right after I put a half-thawed zucchini slice in. I wonder if her skin just gets irritated more easily?