black moore is acting very strange

Question
Well, let me first describe the tank situation for the past week before telling you about the Moore's problem. I have a 29 gallon tank with a 30 gallon filter (Marineland w/ biowheel), some black gravel, and a couple ornaments that have been in there for a few months. My pleco and celestial goldfish died for unknown reasons in the past 8 days. No signs of any disease or parasites at all; Plus, my girlfriend who works at a pet store tested the water for everything and it was all o.k.. I did about a half tank water change yesterday(used stress coat too) and also tested the water again(was pretty much perfect for ammonia, pH, nitrates, etc...).  

Today, I get home to find my big Black Moore(about 2 inches tall torso, and 4 1/2 inch long body w/ no tail included) was lethargically laying on the ground, but my little Black Moore(about 2 1/2 inches long total) was fine. Tested the water for myself and the ammonia was 0 and pH was around 7.5.
Did another partial water change(about 1/5 tank) and the pH is around the same. He seemed to be a little more active after the change since the water out of the tap is a little warmer than the tanks but he's still been laying around for the past 7 hours or so. What could possibly be wrong with my tank and/or fish?

                              Thanks, Todd Clark

Answer
That is strange. There air and a heater in the tank? If so, is his feces white? If so, it is possibly Hexamita which is treated with medicine that has Metronidazole in it. Oscars are notorious for getting Hexamita.
Also, stop doing all those water changes. It is causing undue stress on the fish.