star polyps and silvertipped xenia problem

[StephNewman]

For some reason my silver tipped xenia is melting away and my star polyps also seem to be closing or deteriating, everything else in the tank seems to be doing fine, can anyone possibly know what could be causing this.

 

[scubabill]

too much flow????

 

[SSTG]

Hi iodine?

 

[StephNewman]

I don't add any chemicals to the water except for pH buffer so not sure how the iodine could be high. I test calcium, phosphate, magnesium and all of them are within normal. PH, ammonia, nitrite are all notrmal. Only thing off is Nitrates and that is always off at about 20ppm. I have never found away to keep that low.