Yo,welcome on board!
Regarding the centipede, i think it's called Bristleworm. Should not posse a thread to your lifestock, they may damage Corals but i dun think fishes are affected by them. THey have beneficial effects to the sand-bed as well. If yours is a FOWLR tank, you may wanna get some worm eating fishes to finish it off. If not, find it's lair and use a chopstick. (that's what i did)
As for the fish dying, i think more details on water parameters and etc will help