If it has crud to stir up it is dirty bud. Get yourself some of The Works toilet bowl cleaner for 2$ and slosh it around. It has a really high acid content for anything you can buy over the counter without some kind of license lol
Take off your petcock and plug the hole for the job otherwise it will eat your petcock. Agitate, sit, stir, all that good stuff, get it to clear out the crap. There's other solutions but they are more time consuming/work/$$. This is down and dirty but effective. Afterwards you absolutely have to get some type of viscous fluid in there like ATF, 2 stroke oil, something to coat the tank so it doesn't immediately flash rust.