Some first-hand experience, I'm using PhosGuard for almost an year, LPS, softies, mobile invertebrates and fish.
It depends:
- in the tank with moderate phosphates production it was very efficient, solved diatom problem in a few days, without return (it removes Si too),
- in the tank with very heavy bioload and high PO4 production, it's less efficient, even in double dose (could be because there is some speed of binding), and gets exhausted in a few months,
- the only coral, that reacted on addition of the PhosGuard, was one of the chili corals in period of the fast growth after recovery. Could be coincidence, I made massive water change and added micron filtration and carbon. Didn't risked the second time with either of these factors. The other kinds of chili (I have 3) are without problems with PhosGuard.
- used it for a few weeks in the tank with baby Maximas (3/4" Tridacnas), no ill effects were seen, grow beautifully.
Just in case ordered RowaPros for this chili coral tank, but lowering pH is a significant risk too.