We use TSP (Trisodium phosphate) available in the paint dept at home
centers. Use the REAL TSP not the "substitute".
Using laundry soap is asking for trouble (BUBBLES)
Let it soak overnite in a TSP solution: 1/4 cup TSP to 2-3 gal water
Then using the strongest 'jet' of water you can get out of your hose, spray
it off getting between the folds too.
We keep two filters, one in the tub and the other, clean and ready to drop
in so you can do the overnite clean/soak above on the used one