It is a lanthanum chloride solution that renders phosphate insoluble. It is a much more temporary fix than any of the others suggested. Think of it as a very quick fix, as it is far from a permanent or long lasting solution to phosphate. If not removed by very efficient water polishing, it can also become soluble again. If you are looking for long term control, the cheapest method and one of the most efficient is to use algae. Many do this in the form of the Chaetomorpha alga, while others (much fewer) try things like turf scrubbers, etc. Another method is to use GFO, which adsorbs phosphate ions to its surface and is a good long term solution--not to mention that it doesn't carry the same side effects of aluminum oxide, which has been shown in numerous studies to cause problems (actually more so for vertebrates than invertebrates). The last option I recommend for beginners is to use organic carbon-based additives. The method can work well, but it has a number of setbacks and expense issues, not to mention a suite of other unknowns that I, myself, prefer to avoid. You can look into this method in more detail, if you wish, but I prefer to stay away from it. The tanks that use it look good, but most of them have hefty, efficient, and expensive protein skimmers running them, hence the added expense (among with the suite of other optional supplements that are often recommended).