Green button polyps inside out

[WindeyD]

I've had these palys for 6 months and they've been doing well, about 3 weeks ago they turned inside out (see pic) and have remained that way for weeks. My parameters have remained stable with nitrate<5ppm, cal 500, ph 8.3, alk9, salinity 1.025 and temp at 78. No changes to equipment have been made during this time. They are still budding new polyps and all other corals are thriving; any input out there?

 

[Themonsterisme]

What's your phos and nitrite at?

 

[WindeyD]

Both are 0

 

[joshporksandwich]

How do you keep your phosphates and nitrates at 0?

 

[WindeyD]

Nitrates are actually <5ppm to zero; I have a very low bioload and a skimmer rated for more than double the tank size; as for phosphates my testing reads undetectable though I've read these tests can be unreliable but I have not had any signs of nuisance algae (knock on wood!!)