aluminum sufate is effective for phosphate control and is also used a a floccing agent in pool. Don't use it unless you can vacumn to waste! If you can't then the lanthenum chloride baseed products are a better choices since they will keep the phosphate precipitate in suspension in the water and your filter will remove it in a few days to a week. You need to watch the filter pressure and clean it as it rises. That is really the only danger to your filter. If the phosphates are very high the pressure can rise very fast and in worse cases the filter could need cleaning a few times a day. DE filters are probably the worst offender here.