gas tank sitting for long time

[yellowGT]

I need some help from you guys. I decided im going to use the behind the seat gas tank so I pulled it out because by the smell of it you could tell there was old gas in it. After pouring that old gas out I noticed a black sticky substance on the bottom of the tank. I have no idea how to take that stuff out. Im guessing some kind of solvent that I can slush around but im clueless. Any idea?

I found the sending unit pretty cheap so im replacing that too.