Ok so my buddies 75 gal tank keeps killing fish. All levels are perfect ph ammonia nitrates etc. salinity is spot on and all his equipment is in great running order and is all top notch quality. Well as close to top notch as one can afford lol. But it's not cheap equipment by any means. All his corals are great and healthy and I'm running outta thoughts as to his issue. Stray a/c current and possible lack of o2. Anyone have any ideas. He called me about an hour ago w all his fish hanging out at the bottom gasping for air. I had him float some ice to make sure temp was right at 76 as it was about 78 which is still a good temp and also upped the o2 levels. His tank is definitely established and doesn't have any diatom or other algae issues. Fish don't have any parasites or ich that is visible and he never jumped the gun by rushing anything as far as stocking it or overloading it. I'm just outta ideas here. Any help would be appreciated. He's gonna grab a ground rod tomorrow as per my last guess of his issue. Thanks