So I noticed that my Sunsun 303B canister output was not putting out much water. I replaced the spray bar with a adjustable dual return nozzle w/flared ends from eBay a year ago. Spray bar restricts too much water flow but this dual return really lets the water flow out.
Anywho, taking the Sunsun apart, verifying the impeller is just fine and putting back in action, still slow water flow... Hummm.
So I pull off the intake which has my coarse pre filter on the end and the water now shoots out the output! Well clogged intake pre filter. I just put on another and all is back to normal.