Have you check for any stray voltage in the tank? Maybe a long shot but i know of 2 reefers and a LFS that had that problem. Voltage wasn't high enough to feel or kill fish but sps wasn't happy.
1 had the bad jabeo pump. 1 heater only when water was over the top.
Test with everything on, jiggle any cable thats in the water, check ATO switches, etc
Test by using a voltmeter in the AC setting and test at at lowest scales.