Problems with Parazodanthus

[Victor]

Hi I'm a beginner in this hobb and I have my first problem with corals, in my tank of 500 litres I have diferent corals (Actinodiscus, Fungia, Tubastrea, Sinularia, Rodactis, Jewel Coral and Parazodanthus). Every coral was OK, but suddenly except Parazodanthus. I have 2 big colonies of Parazodanthus and some little ones. Except one of the big ones, the another ones they are closed for some days 10-12 days). I don´t understand why, I can see no parasit, I can see no illness, but they are closed, but not every colony. No fish it´s attacking them. I have no idea what happen, parameters they are OK, only I had recenty high nitrates, but now it´s OW. Any idea? Any solution?.... I would like your help

Thank you... and excuse me for my english, write from Spain:confused: :confused: