Clean them by:

Spraying them off -- NO pressure washer; no scrubbing.
Soak them in 4 gal water + 1lb TSP + 1/8 cup of bleach each time you change cartridges.
When swapping, remove from tank; drain, rinse, drain again, rinse, drain again, rinse a 3rd time; install

Rotate them regularly, with one cartridge always soaking. NEVER acid clean a cartridge unnecessarily; NEVER acid clean without using detergent (TSP) first.

(I think it was Waterbear who wrote the above. I copied it and pasted it from some notes.)

Also --- sounds like you have a suction side cleaner. Does it attach to the skimmer in some way?