LOL, CANNON!
I take it the lettering is ON TOP of the finish. How long has the finish been on the tank? Is it urethane base/clear?Is the lettering itself lacquer or urethane paint?
If the finish on the tank is fairly old, and the lettering is new lacquer, you should be able to wash it right off with lacquer thinner, but be careful not to leave it on for long, as it can soften the finish. The same should be true if it is urethane, but you may have to get some urethane reducer to cut it.