P04 0.01-0.03 not totaly sure (salifer test kitt)
That level of PO4 in a tank can suggest not enough nutrients are getting in. From my experience that level long term can kill corals, so my preference is for PO4 to be in the 0.08-0.15 range, lot safer. Reef experts can keep corals at 0.03 levels but one needs experience and thriving reef tank.
Also large water changes can help but due to swings they can cause issues. I prefer 10% water change done once a day for a week instead of one large water change. Try to keep things stable…
Also if you are using GAC I would suggest 40ml per 200L of water, changed once every 4 weeks. From my experience too much can stress corals.
Out of curiosity how is your GSP coral doing?
I think I seen one in the pictures. If it is open and alive I believe you can save some of the other corals. The ones that are not dead…
GSP is very tolerant coral so if it is alive things are salvageable.
Anyway I would feed your fish using high quality pellets. My preference is Fauna Marin soft Multi mix. I feed 4 times a day… Nice plum fish (not paper thin) is good producer of coral food.
I would also consider dosing some carbon source to feed the bacteria in the tank. I prefer TM NP Bacto Ballance at like 0.2ml per 200L, not for nutrients control. The idea being the bacteria will be food source for corals and snails also enjoy the biofilm on the glass.
I would also feed coral food once a week. I like FM coral dust but people like Reef Roids. Sparingly, I observed with coral food less is more. Too much and corals do suffer.
I would repopulate with easy to keep corals like soft corals devil hand:
Zoas:
Maybe easier chalice corals:
To build up coral mass and hopefully make the reef tank stable.
If your GSP is dying that is beyond my experience level…
Good luck,