There are commercial cart. filt. cleaners on the market that have worked for me for years. As above you soak overnight then hose off. If you have high water pressure as I do ( 125#) all the better. My filters always came out nearly as white and clean as new. For iron stains - I would very briefly dip them in a very mild solution of acid but that was rare. I don't think the iron stains inhibited the flow. One word of caution, tho, never do the acid solution first. That will ensure your filter will permanently be plugged and virtually destroys them as the body oils and suntan lotions solidify. I have often been asked by colleagues how I got my filters so clean going so far as to smell them.
I used an old S. S. Purex DE filter tank for soaking.
Ken B