Phosphate RX is in essence Lanthanum Chloride... It is a highly effective phosphate "binding" agent that removes the soluble phosphate from the water column. Removal of the bound phosphate is done via skimmer or sometimes a low micron filter sock. This makes sure it doesn't enter the main tank where the livestock is.
If it is dosed directly to the tank in an amount large enough to drop the phosphates too quickly, the Lanthanum will precipitate out too much phosphate which can cause the precipitate to get into the fish's gills causing issues or death. Also there is a correlation to zebramosa tang death and the use of Lanthanum... Nothing scientifically proven, but there has been anecdotal evidence that it adversely affects their ability to breathe...