Fish keep dying

[CPLHESS]

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

[Mike]

How much oxygen? If they are gasping for breath, look at the surface of the water. Is there a film on the surface of the water blocking air dissolving into the water?

[CPLHESS]

No film present on water. He's gonna throw a air stone in refugium this morning. Skimmers working great and should be providing more than enough but air stone won't hurt either. He doesn't over or underfed and the food is rods so it's not like they're malnourished from cheaper foods either.

[Big_Pun]

maybe he should run carbon or a poly filter sounds like a contaminate of some kind basically poison.

[Mike]

How is everyone doing today? Any idea how PH or alk is? Could it be too high a swing between day and night? Doesnt sound like temp is the problem. Carbon or something to remove contaminates couldnt hurt...

[CPLHESS]

Has two stage brs reactor w gfo and carbon refugium has polypad after refug rubble. Ph is 8.3 he's gonna use my multimeter and check water for voltage and unplug one item at a time to check corresponding voltage levels. I'm almost certain it's stray voltage. It's always something simple lol.

[CPLHESS]

Thank you for everyone's help all suggestions are welcome ;)