I have had 2 colt corals and they both died within 4 weeks of being introduced into the aquarium. They look healthy for about 2 weeks, then start to droop and then begin to slowly die. I have checked my water and there is no ammonia, nitrite or nitrate. I have 2 chromis, a scooter blenny and clown and they are doing fine. I keep the aquarium temp between 78 & 80.5 F. Any ideas what cause them to die?