Some sort of mild abrasive,I guess.
Try a tiny amount of T-Cut on a cloth and go gently bently,...and only on the stains.
oooooh this is so open to abuse, but i,ll refrain.....
You'll have to excuse my ignorance as I have a black plastic tank but is the tank painted
Have you try Flash for special surfaces - removes all sorts of stains and evil substances
Try Halfords piant serfice cleaner . spray it on and wipe it off , with no scraching .