We have checked all our water parameters over a week ago and everything was perfect. Our hermit crabs and now our blue hippo Tang has died. We have only had him about 3wks. He was 100% fine last night when we went to bed. Turned on tank and he's dead. He had been eating pellets, algee flakes and shrimp. Only fish still alive is our original clown fish. Corals are doing ok. Don't understand what keeps happening