To do it right and nice you need to redo the tank.I usually strip a tank but the easiest way for you would be a piece of 80 grit on the DA.Once the decal is gone grab some 180-240 and sand out all the 80 grit scratches and feather out the paint.Prime it,block it out to level the featheredge and prime it again for the final sanding.Blow some color on it and clear the whole tank.Anytime you sand through the clear(or pull the decal in this case) you'll see a discoloration between the original base and original clear,I call it a halo.You have to put color over the area and blend it out so the new coat of clear doesn't magnify it.The priming is for adhesion to the metal if you sand that far and to fill in the material you removed back up to the level of the surrounding paint.
