how to flush your fuel tank

I have a late-model cruiser that I recently had a local shop go through the carbs because it was leaking gas out of the overflow tube and not wanting to idle. They replaced the needle and seat and while they were at it they said the tank had water in it so they pulled it off and flushed it out as well. The bike runs great *still has fuel leak from the overflow intermittently even though they went through the carbs twice. As I was pushing it into the garage with the engine killed it sounded like some debris of some type may be sloshing around in the gas tank. The tank is almost empty and it just sounded like something is in there…not sure if it has some weird baffling or something that’s throwing me off of not. I'd rather not have to take it back to them AGAIN. I may pull the tank and drain it to make sure everything is ok. Are there any particular products I should use to flush it out real good? There’s no rust or anything like that…like I said the bike isn’t very old.