Order the Penguin RFUG kits to get the prefilter sponges at least - the kit is cheaper than the FilterMax kits locally for me. It is coarse, but only fairly fine stuff gets through. If that does not do the job, step up to FilterMax. If you are still unhappy, add, don't replace the sponges, the HOT Magnum, and prefilter it as suggesteed. But keep the HOT prefilter really clean to keep from starving the pump (rinse every 2nd or 3rd day). The HOT, when used with additional mechanical (the powerhead sponges at least, or those plus prefiltering the HOT) will keep the tank diatom-clear.
If the p/h pump is cavitating, it means it is starved for flow - wrong sponge or clogged sponge such that the impeller is starved.