With the 0 ammonia confirmed, and your feeding regimen looking pretty good, that leaves the heater possibility and some unknown toxin possibility. Moe did some work a number of years back that showed that under some conditions common Vibrio sp. bacteria could pump out toxins under dense culture situations. If the heater does not pan out, it might be wise to set up a fresh tank with a new batch of aged SW and transfer the fish. Actually even if the heater does prove to have a problem, a large water change would be a good idea. Also keep in mind, the damage from stress could take several days to recover from after the stressor is removed.
