So popeye is a bacterial infection. It's usually secondary to something else... an abrasion (cut), parasites ect. So the first thing we need to look at is... are any of your other fish displaying symptoms of parasites? Have they ever had ick or flukes? I would mention velvet but they would probably all be dead and you might have mentioned that already lol. Think about rubbing against the sand, rocks or powerheads even. Twitching their heads, being reclusive and hiding alot, white dots on the fins or body (even just a few and even if it was weeks ago), swimming into the flow of a powerhead alot.