Like everyone said: vinegar.
There are nearly no other chemicals that can be used to clean a tank. Soap will leave residue even after a dozen or more rinsings, and this residue can kill the fish.
Oddly, the only two other chemicals that I would suggest for cleaning a tank are two of the most toxic chemicals many of us have in our homes: bleach and ammonia (though never at the same time or in the same tank). As long as they're pure, dye free, and scent free, bleach is easily taken care of with an overdose of dechlorinator, and ammonia will be taken care of by cycling the tank and doing water changes.
I only suggest these two, however, if someone has a tank that has had soap or some other chemical touch it. It's about the only way to really be sure you get rid of soap residue.
As long as you haven't put soap in the tank, stick with the above suggestion of vinegar.
