How to clean a fuel tank

[theoldwizard1]

I suspect I have some contamination of some kind in my fuel tank. Other than simply draining it through the pickup (gravity/siphon) what else can I do to make sure my tank is REALLY CLEAN ! (Yes, the tank is in the boat and not coming out !)

 

[Dave-R]

My boat boat had been underwater for 8 months. I had good access to the fuel sender, which is about a 1 1/2" hole in the top of the tank. I borrowed a inline electric fuel pump, and hooked it up to the normal fuel out let hose. I pumped out 50 gallons of not bad looking fairly clear stuff. I then took a coat hanger, and bent it straight. To the end I tied on as big of piece of soft terry cotton rag as I could get down the hole. I was able to get everywhere in the tank. Then I filled about 10 gallons of gas, and fuel stabilizer. Then I took it for ride behind the truck . I drained this and used it for my riding lawn mower, worked great. Dave-R

 

[slag]

Taking it for a ride was a good thing as it probably made its tummy upset and release the rest of the bad stuff. Good idea.