I used some a long time ago when I found that I had a measurable amount of phosphate. It worked. not sure but I think it somehow binds to the phosphate and allows the skimmer to remove it.Ever since then (more than a year) phosphate always tests 0. Within the last month I noticed a couple small patches of briopsis or hair algea and still 0 po4 so I put a little in just in case the po4 levels are below my test kits range but high enough to sprout some algea.