I think we are overlooking the main problem here. Your lighting dose not seam to be enough for that coral, nitrates should be at 0 for sps
corals. within a 5 gallon tank there is bound to be chemical warfare between corals, thus needing good flow and carbon. You do not need to be changing water every day. Try once or twice a week and only about 10% at a time. I have a feeling that your tank has not been set up that long. May I ask how long it has been set up for? Also it is very easy to over feed in such a small tank.