Plate coral almost dead in a week

[Localmandave]

Hi, I have an issue I am stuck on, about 2 weeks ago my 450 litre reef tank of about 3 years started to look like corals not at there best, 1 of my hammers was not opening fully like it had, then I noticed my plate was starting to look like it was peeling and an unknown was not looking healthy and shrinking in life, checked water and nitrite has increased slightly but all other params look to be ok, ca 440, MG 1680, sg 1.026, ph 7.8, kh 8, I have done a 25 litre salt water change, fish are all looking healthy and only corals affected, lights are hydra 26hd and on from 10am to 10pm including ramp times, I have a skimmer and uv, any help is appreciated and hope I have given enough info, ask for more if not.

 

[Shorething]

Phosphates?

 

[Double monti 61]

Maybe not enough food?

 

[Localmandave]

Phosphates?

Did test and 0 for phosphates

 

[Localmandave]

Maybe not enough food?

Been feeding every other day

 

[Shorething]

You said your nitrite increased? Did you mean nitrates? And what was the value?

 

[Localmandave]

You said your nitrite increased? Did you mean nitrates? And what was the value?

No2 rose to 0.037 ish and No3 is stable less than 0.5

 

[Localmandave]

Any ideas anyone please? Thinking about a 50% water change and additional filter