TSP if you can get it in your area is one of the best things to use to soak them 1 cup per 5 gal water. (electrolsol and cascasde are basically just TSP or one of the non phosphate replacements for it, depending on whether phospates are totally banned in your area, with some other ingredients added) If there is scale deposit visible after the soaking then use the acid bath...once again 1 cup muriatic per 5 gal water. NEVER soak a cartridge in acid until you have cleaned all the oils and organics out of it by soaking in TSP or you will harden them into a cement-like substance and will need to replace the cartridge. Some 'old time pool service guys' would combine the TSP and acid in one step which seemed to work and is pretty much what most of the commercial filter cleaners are...a degreaser and acid.