I split your post so you can specific answers.
The best advice is determining the source of the PO4
. Once that has been determined and eliminated usually source water, foods and sometimes cheap carbons. Then the rest is relatively easy.
If using RO
water, many times some twice weekly 10% water changes will do the trick. In harder to deal with problems you could also look into phosphate resins but I personally would't use them if there are corals in the tank. They arely react well to them.
Post once you get the test done, that will help alot.