Big Water change?

All of my water params are fine exept Phos that is sitting at .5 I have been trying to get it down for weeks. I have no idea where it is coming from,

Tested new water, Ive been doing small water changes, I cannot get it to budge. Tank has been up 2 years.

My Zoas are completely Closed up have been for days, my brains look like they are dying, showing skeleton, everything is going down hill.

I am running new carbon and new GFO

I have a skimmer thats over rated for tank also

Im thinking that about a 50 % water change would help with brand new water from the LFS which I have tested and is perfect. Thoughts?

I also dont have an algae problem, which makes no sense to me

 

are you using a reactor for your carbon and gfo?

am asking because i had phosphates of 0.50 and no algae out breaks. ( until i added a new MH bulb, but before that all was good... so high phosphates can cause problems without algae out breaks, the out break is why i checked the phosphates than i seen why things weren't doing well)
i purchased a brain and it didn't do well turned into a skeleton. corals were struggling... i was running my carbon and gfo in a high flow area in my tank and not in a reactor. after adding the reactor and putting my carbon and gfo into it. i seen a huge difference, water sparkles, phosphates dropped every other day when i tested i seen them dropping. corals all opened and swelled ...every thing looks so much happier . phosphates were 0.07 ( hanna checker) last i checked and that was over a week ago maybe 2 weeks?????. so I'm guessing they are near zero by now.

 

+1 GFO has to be tumbled to work correctly. Fast flow, and it can't collect it properly.