Ben, the 5% CAME from Chem geek! It has been proven to be effective in a vast majority of salt pools, possibly because of the super shocking that occurs in the cell. (In fact I am not familiar with one case where there were algae problems with someone following it and making sure that FC did not drop below that but,as we know, there are always exceptions to everything!) Also, remember that 5% is the MINIMUM FC level you should run.