Other than that chemical...
3 gallons of vinegar, 1 cup of salt, and a 2 gallon jugs of water with a cup of baking soda dissolved in each of them is all you really need. $20 or so.
1. Empty the tank, seal all the openings (or leave the petcock in)...
2. Put 1 gallon of vinegar and 1/3rd cup salt into the tank. Seal the tank, and shake the ba-jesus out of it. Let sit for over-night (10 hours) upright.
3. Shake tank again, pour out vinegar-salt solution. Re-fill with 1 gallon of vinegar and 1/3rd cup salt. Shake it again. Let it sit up-side-down over-night.
4. Repeat step 3, only leave it standing on end (back of the tank facing down).
5. After 3 nights, 3 re-fills and a bunch of shaking... put 1 gallon of HOT water with a cup of dissolved baking soda into the tank. Shake it... pour it out. Repeat with another gallon of HOT water/baking soda solution.
6. Rinse out tank with a bottle of 95% Rubbing Alcohol.
7. Spray the inside of the tank with WD-40... go crazy, empty the whole can if you need to... but make sure every square inch of the inside of the tank is covered until you're ready to use it.
If after Step 4 you still have a lot of rust... you'll need to repeat step 3, but before you seal the tank, put a length of chain, nuts and bolts, or ball-bearings (bb's are too small) into the tank... about a hand-full will do. Then shake it till the cows come home, let it sit for an hour or so... shake it again, let it sit in a different position for an hour... and repeat. Check the progress after 3 or 4 rounds of shaking and letting it sit.
This has worked for me on everything from in-frame moped tanks, to motorcycle and automotive gas tanks.