I was looking for some additional info on electrolysis and ran across this article. http://myweb.tiscali.co.uk/andyspatch/rust.htm
It gives quite a bit of info on the process and also gives the details on a current limited power supply you can build.
I plan to build my tank soon and have been collecting parts, and like Rudi my cost is at $0 so far. Will keep the forum posted as it progresses