You could drain the tank and try to press it back out from the inside, but personally that seems like more trouble than its worth. I punched in the dimple on an aftermarket skid plate which did the same to my tank. I just fixed the skid and have been using it. It's possible that the weight of a full tank of gas over time will restore the plastic tank to its original shape. Next time I have the tank down I'll check but not something I worry about.
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