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.
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