Brittle stars can eat fish, but I doubt one brittle ate all of the fish in that period of time. I also doubt that any one predator took out all of the fish in this period of time. My guess would be that you have gotten some sort of fish disease brought on by the stress of the move to the new tank...
32 gallon hex, 34 lbs lr
, Prizm protein skimmer, fluval 104, 1 maxi jet 750, 1 maxi jet 1200 ph
, current 40w 50/50 pc
, 1 NO
powerglo 19watt, 1 daylight NO
65w (equiv), two damsels, 1 green brittlestar, 1 peppermint shrimp, turbo snails, red legged hermit crabs, kenyan tree coral, leather coral, pumping xenia, mushroom coral, green star polyps, orange button polyps