Freeze spray is another option.
Put the part in the sun or heat with hair dryer first.
In the case of a tank, drain it first.
I talked to a guy last night who has taken hail dents out of a car hood by putting it in the sun and running cold water on it.
But, a car hood and a motorcycle tank are different problems in a couple of ways (thickness and contour).
I just got done trying heat gun/freeze spray on an old Triumph tank.
No luck.
Definitely worth a try, though.
