I use TSP (Trisodium phosphate) that I buy at Home Depot or the like. I use 1 cup tsp mixed with 5 gallons of water in a bucket, let is soak over night, then rinse and soak in clean water the next night. Because my HS spa has five filters, I have a 6th filter for soaking and have them on a rotation schedule. In the winter I can't easily hose them all out at once, so I take the filter from the main circulation pipe because that's the one working all the time. I just keep shifting them in a circle so that they all wear about the same. So far so good and I find it much more manageable than cleaning them all at once in the winter. Now summer is another story, I still rotate the clean soak, but I can easily spray them all out every couple weeks. My year and 1/2 old filters look brand new with the exception of the bottom and top plastic that has faded.