Moped Army

i have discovered that i have a reaky nasty
rust problem inside my "built-in" fuel
tank.I, unfortunatly dont have access
to that acid stuffthats normaly used
bacause i dont have a credit card
(only 15 y/o)
Can i just use water, because
old gas with the rust formed this
sludge inside the tank.
Also, when cleaning the
petcock, i noticed that
it was all gummed up by
this black clay stuff.
What is that?
I cleaned as much out as possible
with carb cleaner, and a tiny screwdriver?

can you guys help me out again?