Dont add fish to clean your tank, it doesnt help.
Step one, gather your rocks. dont use any "soft" rocks like pumice, limestone, shale or anything you can sctartch with a nail. Step two, scrub the crap out of the stones under running tap water with an abrasive pad. Step three, boil the rocks for a few hours to kill any micro organisms that could be living in the pores or cracks, than add your rocks.
The reason I say dont add fish to clean your tank is because fish eat, than they crap. Poop is bad for your tank, it raises your ammonia and causes your tank to need cleaning moreoften. as all you have is a 20g hi ammonia spikes can happen very easily. Your tank is fully stocked now. If you realy want to add ONE more fish get a cory cat or something else that stays SMALL and at the BOTTOM. even better yet those blind cave catfish.
If you want PM me and I will give you some tank cleaning tips
Quinn