I am in the process of cleaning my tank also. I'm using a three part solution that lceans and protects your tanks. its called Kreem. You can get ti at most motorcycle shopes. The first one is a cleaner solution that acutally disolves the rust and give the tank a gret appearance inside. Then there is a prep and a sealer that you use. This will make the inside of your tank clean and rust proof and if you have any small leaks it'll seal this too. Right now I have the solution etching overnight and its already showing signs of greying. Should be done by morning. This kit is about $30 but can do prety big tanks. Over 5 gallon tanks and car tanks.
The nuts thiung I'm sure works too. Actually on the bottles they say that if you have a large amount of rust to use nuts or stones first. I only has light surface rust and no flakes. So i jsut went to the etching.
Hope this helps