I am at my wits end. I have a 265 gallon FOWLR and started the tank March 2008. Had about 15 small fisn (yellow tang, cardinals, etc) up until December. I added 3 fish that were not quaranteened (I know already this was a bad thing to do). 30 minutes after the fish were added, the bigger fish all died lying on their sides and the small fish were all gasping for air at the top of the tank. All fish were dead within 24 hours. Ammo and nitrates were 0 but nitrates were 80. Over the next two weeks I systematically changed over half the water. When all readings for the water were safe, I added 7 more fish (tang, blenny, small angel, etc). Within 3 weeks all these fish were dead. They died one at a time with no apparent signs of aggression or disease. I left the tank running empty (except for the rock and some inveersts) for 5 weeks. I added a yellow tang, a small lion and blenny this past Saturday. The lion and blenny died in 2 days and now the tang is flashing. The water parameters are normal and no apparent signs of disease on any fish. Besides not using a Q tank, can anyone tell me what is going on here and why I cannot keep any fish alive in this tank? Better yet, how can I fix this? Please, please help. Any help is appreciated.