Fish Dying Biocube

I have had a biocube 29 up and running for about 2 months now. I had a pair of ocellaris clownfish in there for a month. I recently ordered a RW4 Powerhead and added it to the tank to help with the circulation for my corals. The very next day one of my clownfish had died. My guess would be is that the powerhead stirred up some of the sand and it released toxins into the water that caused the fish to die. I did a water change and everything seemed fine but now my other clownfish has died a couple of days later and I can't figure out what is going on. I cycled the tank for about a month with 15-20lbs LR and about 20lbs of LS. All of my parameters look to where they need to be. Ammonia and nitrites are at 0, PH is at 8.2, salinity is 1.025, Phosphate 0, Nitrates are about .5 and my temp is around 79. My Alk is also looking good. I am currently using a self made media rack out of eggcrate with filter floss and chemipure elite.

Does anyone have any suggestions on what could be causing the fish loss? I have a star polyp, torch coral, frogspawn, and a zoa that all seem to be doing fine. My CUC consists of 1 peppermint shrimp, 4 hermit crabs and 5 nassarius snails which now are the only things in the tank.

Any suggestions would be great. I am obviously going to hold off getting anymore fish for right now until I can figure out what is going on but I dont know where to start since my parameters seem fine.