Ok got a fuel tank that got a dose of water in it and has started rusting.
Was going to chuck some rocks in and shake it around abit to clean it up but any ideas of how to treat it to stop it from starting again?

Take it to a Radiator shop for cleaning

http://www.ppc.au.com/webcontent20.htm 

use chain not rocks, then you can ensure that there is DEFINATELY nothing left in there.... wash with white vinegar and soak it full of the vinegar,, it will strip the rust and wont kill the metal too much....
wash it thuroughly then use a tank sealer

Like Dave said, if you want to permanently stop it from rusting again, you need to seal the inside. The POR15 Fuel Tank Repair Kit is excellent value and you can do at least two Beetle tanks with one kit. For $100 plus S&H, it's probably cheaper than cleaning at a radiator shop.
Look here http://www.ppc.au.com/category33_1.htm

Hey Nath, I just bought a POR15 fuel tank sealer for my beetle, and seeing that i will only need half of it, do you wanna go halves? Gimme a u2u if you're interested...

PurpleT3 hit it right on the head, good call. It's not cheap but worth the $$. Just don't tell the wife.

Everyone beat me to it.
Go the POR15.

Cheers
Dave

yeah i've used the POR kit, its not cheap but its not that bad. I'd probably prefer to take it somewhere to get properly dipped though, the POR kit doesnt really remove the rust, it just converts it and puts a zinc coating on it.

The vinegar will remove the rust and clean the metal.... if you would like to see proof ill get the camera out.... youll have a spotless tank then you can prep it and POR-15 it

Going the rock cleaning method first and then the tank sealer.
Its only a small tank 7lt Its from the spare wheel in my oval.

Nathan

use chain not rock, then you can be sure there is nothin in there when you tip it out