How do you lower the phoshate?
Test for it.
Test everything to determine where it's coming from.
Use phosphate free freshwater for saltwater mixing and evaporated top offs. RO or RO/DI water is great.
Don't overfeed.
Don't use cheap activate carbon
Drip kalkwasser
Use phosphate sponge media/pads - change often.
Utilize a macro algae refugium and harvest/remove algae.
Siphon detritus from the sump.
Test for it often to see if the changes you've put into place are reducing it.