If you are concerned about the health of the sps in the display tank, take the most infested rocks out and put them in a small QT tank with your tank water. Apply treatment in that tank (Mg or AlageFix and lights off or whatever). You can slowly go through all the rocks with this method. In your QT tank put the rocks under high flow and use a phosphate absorbing medium to soak up all the phosphate. Change water, let settle, then put them back your tank. Do the phosphate sponge in your display as well. After a while you should have killed off the existing stuff and solved the root problem of high phosphates without putting your corals at risk during drastic chemical/biological treatment.
Nick