Sorry to hear about your loss, if it helps this happened to me about 2 years ago in my 2 year established mix reef nano tank. One day came home and the 1 year + clown fish, bicolor goby, 6line, Cleaner shrimp,and mandarin were all dead. Only thing that lived was the larger of the 2 clownfish....and he looked 100%. All 50+ pieces of coral fine, EVERYTHING and i mean all tests, (salinity, phosphate, nitrate, ammonia, iodine, iodate, ca, mag, Kh, Ph,) perfect. Could never figure it out. Only guess was some stray current somehow. Who knows, good luck with everything!