The problem is in the drainage tube, disconnect the drainage hose from the Dip Pan side,attach a Dry Vac to the hose (use duct tape to make a tight connect). Have someone cup the outlet side
of the hose to feel for suction. If the suction is low the blockage is in the hose (Algae,sludge). Block the outlet side to create more vacumm in the hose. Eventually all the sludge will be vacummed out. Good Luck.