Fish died - any help or ideas?

[snailbuddy]

This tank is a 20g reef 4 months, pc lighting, 2 filters, 4 powerheads, 20+ pounds live rock, substrate, 2 small ocellaris clowns, 1 bi-colored blenny, a collection of small corals, 2 cleaner shrimp, 2 fire shrimp, 1 emerald crab, 4 tiny hermits, several snails. I only use RO water, do 10% changes once every 2-3 weeks. I fed the tanks and noticed a clown in obvious distress. I tested the levels, all perfect except for nitrates - 80 ppm. Did a 25% water change, nitrates down to 40, fish still in distress. Couldn't find the other two fish. By morning, the clown had died, still can't find the other fish. All inverts and corals are doing great. No sign of disease on the fish. What the heck? OK, I know I've been over feeding - bad, will stop - feed frozen brine, mysis, etc. Use marine snow, combo vital, plancto. Any ideas what caused the fish to die? Am doing another 25% water change today. Thanks for any help!