NOTE: Applying stickers to plastic fuel tanks:

All decals bubble on plastic tanks hence why they come with holes in the tank stickers to help minimise this effect.

This happens due to the fuel inside giving off vapours especially as it warms up.

Once it cools off again you may be able to rub the bubbles out.

It may help keeping the fuel tank full.

The only solution to stop this is to not have decals on the tank at all or use a metal tank.