If you're cross threaded you can pursue a couple of options. The first thing I would do is unscrew the fitting and visually inspect the threads of the tank.
I have seen instances where the threads were cut at a slight angle. This looked funny and is a reflection of poor craftsmanship, but is still functional. If yours is cross threaded and does not appeard to be badly damaged you can try to re-cut the threads as opposed to over tapping.
Otherwise...you will need to see if the tank can accomodate the next tap size and install a larger fitting, or goop the threads as mentioned in your post.
Plastic is fairly forgiving, and can be easily tapped. Use a good quailty tool, food grade oil for cutting, and take your time.
"One of the best lessons I've learned is that you don't worry about criticism from people you wouldn't seek advice from."
William C. Swinney