You will have an outlet pipe from the bottom of the tank ( discharge ) - a second into the tank from the toilet - a third which is an overflow from the tank - a fourth from the deck
fitting down into the tank ,which is there to enable you to pump out your tank from a shore side facility . This pipe obviously reaches almost to the bottom of the h/ tank and when the crust build up occurs , it almost blocks the main discharge outlet and causes all sorts of problems . Either shorten it or remove it all together .
fair winds Garth Gregory Beneteau