A quick reply to say that my 80cm PA had the same problem last week (symptoms: skin discoloration + peeling off i.e. "burnt" + fish constantly floating) - despite a powerful filter and everything. Thanks to this thread, I identified the cause almost immediately and did an emergency water change. Fishy looked MUCH, MUCH better a day after that (Phew!)
The cause? I myself was to blame as I had been feeding my PA too much (a lot, actually) high protein diet (raw fish + king prawns + dry shrimps) for the past couple of weeks; the ammonia issue had never crossed my mind as I was naïve to believe that the bugs in the filter canister can do "the job". Alas!!
Thanks guys (again)!!