Tri Sodium Phosphate. Can be bought at any Hardware store. Home Depot and Lowe's sell it in the tile, cleaning section. A powder that is really cheap, and a cup will clean any filter. Unicel filters even recommend it to use. That is if my memory serves me correctly. Add it to a five gallon bucket and put the filter in it. Remember to rinse extremely well 3-24 hours later. Leaving in overnight is the best.