Do people read anymore? It clearly says engraved on the HOB filter "DO NOT SUBMERGE IN WATER, MOVING PARTS, RISK OF ELECTRICAL SHOCK"
If you have a media sponge or other Natural bacteria surface, and you really want to give your filters a good cleaning which is actually good for the moving parts that CAN GET WET, remove the cartridge, media and/or carbon elements, and take a toothbrush to the entire INSIDE of unit, the intakes, intake covers, turbines, etc can all be removed and scrubbed w/ a toothbrush or Pipe-cleaner to allow better flow.
For the longer tubes on canisters, a straightened metal coat-hanger covered with a 3-4ft length of nylon legging secured by a series of rubberbands is perfect for this job.
NEVER use any household detergents on aquariums accessories or tanks since they will leave a residue that is toxic and absorbed by the fish. Instead, unscented bleach can be used prior to filling a tank for sterilizing purposes, followed by a 5 minute+ rinse and 24 hours+ to air dry, as the bleach will leave no residue and remaining odors will evaporate.
White vinegar can be used to take care of hard-water stains and calcium build-up