A common issue. If you have decent access and a port to snake a tube in you can wiggle it to the bottom corner and suck the krud out from under the "good fuel". Most often it is the access at the "pick up" port itself that is the easiest to suck out from. You just need to get deeper than the pick up is.
A drill motor pump will work fine into a large container. When it runs clear your good to go.
Do change the filters after your done and say farewell.
Dave Neptune