you could have some thing leaching out of the Live rock you just got and that could be doing this. you could have a dead critter in the new live rock you don't know about. A cuce that is nuking your tank Inverts can be resistant to many things in a tank and will withstand many pets and toxins. Some cleaner got into the tank by accident on the air currents that is why you don't use sprays near a salt water tank. Many salt water critters release toxins in defense of fish so as I said it could be a critter that is dieing that is releasing its toxin or just died. get some Tonga snails they will let you know if some thing is dead or dieing they will go straight for it if they can get at it they will clean it up.