Just curious what caused you to do this... Have you been fouling filters?
Your method will work to some degree, but if the tank has set for any length of time, the debris inside would have settled. It is not the same as have the tank strirred up from the motion of the vessel when it moves, pitches and yawls, causing the debris to float in the fuel
The port you are describing is probably an inspection
port, the best way to use that is drain the tank into a separate clean container and shine a flashlight inside. I know it is a pain, but try looking at as much of the tank as you can. If you see rust or debris, it is time to clean tank.