I cleaned a horribly rusty tank with several bottles of "The Works" toilet cleaner. It's super cheap and did a fantastic job.
You would want to pull the pump and whatever else from the tank first, and block off the openings.
Protect your paint as well, have a water hose handy for any spillage.
After I sealed the openings, I poured 3-4 bottles in the tank, and swished it all around every 15 minutes or so. after about an hour and a half, I dumped it all out, and did several rinses with gas.