My fish are dying and I can’t figure out why

[herbx]

I’ve having fish randomly die in my tank this past month and now I’ve had 4 fish die over the last 2 nights. My parameters seem normal with nitrates being a little high. I can’t figure out what’s killing my fish. The day before Ill notice a fish breathing rapidly and then it dies over night. I have a QT setup but even if I manage to catch the fish and get it in there it does not matter.

120 gal with 55 gal sump
Salinity is 1.027. pH is 8. ammonia, nitrite, phosphate are zero. Nitrate at 20.

Filtration is filter socks, protein skimmer, refugium with chaeto. Reactor with carbon. And marinepure cubes.

Current surviving live stock for now is : 4 clownfish, 1 yellow tang, 1 flame fairy wrasse, 2 blue damsels, 4 coral catfish, 1 ember blenny, 2 peppermint shrimp, and a couple of snails and hermits.

Corals are 3 torches.1 hammer, and a few zoas.

I’m so lost on what to do and what’s killing my fish. I’m ready just to sell this all and be done with it.

 

[DonDrummond]

Marine velvet is a quick spreader and killer so maybe look that up. Sometimes u can see tiny white spots like ich but sometimes it’s just a velvety look to their scales, heavy breathing, clamped fins. Although your tang is still good and they’re very susceptible to it so maybe that’s not it.
Any stray voltage or even longer shot some bug spraying or other chemicals used in the house recently?
So sorry- it’s so sad and frustrating to lose the little guys.