why are my zoa dying

[donkey77]

why are my zoa dying off i have them all over my tank and they seem to be melting away my pari are in check , they were all doing fine then colony after colony are melting away and i thought the zoa were the easy ones to keep
i have good flow a wave box 3 ai sols, 5 tangs, banded coral shrimp, cleaner shrimp, 90g tank 10g water change every week, iam starting to get depressed
i need help
i forgot my frogspawn half of it died in about a day but the other half is fine what gives