All fish died in the FOWLR

Hello,



Over 4 months ago, I started a 55 gallon FOWLR tank since I great success with my 75 gallon mixed reef tank for over a year. I went away for the weekend after feeding the fish on Friday evening and when I came home Sunday evening all of the fish were decease but none of the inverts were deceased.

Background on tank: about 70lbs of Live rock, 20lbs of live sand with about 10 non-live reef sand, Wavemaker with duel powerheads, eheim heater, Coralife 65g Protein Skimmer and a HOB power filter for Chemi-pure bags

List of deceased: pair of Blood Orange Clownfish, Red Spotted Hawkfish, Watchman Goby, small Sixline wrasse, scarlet cleaner shrimp

List of alive organisms: 2 Emerald crabs, Bristle starfish, yellow starfish, urchin, flame scallop, many snails and hermits

Water when I test it was:

Temp: 77.7F PH 8.3, Ammonia 0.00 Nitrite 0.00 and Nitrate 20.0 ppm, Alk 9 dkh

Tank was dosed every week with Special Blend and Nite Out

None of the fish had any visible sign of disease (aka ich)



Any suggestions of what caused this?



Thanks,

Kyle