Would you like an optimistic answer, or the truth? The truthful answer is; Only a professional body shop can remotely repair that tank. Its crushed in too many directions, on edges, and deeply that no pushing, pulling or home remedy will make it remotely close.
The paint on it is now shot too (which is a tragedy as the paint looked really nice!) and even after "repair" will have to be repainted fully. Due to the creases, the metal is badly stretched and the paint won't flex back and forth. In fact, the paint around the damage will begin to peel off and large chips will form as soon as moisture gets under it.
If it were me, I would seek to replace the tank. I suspect a Professional Body shop to charge about $250-$400 for body work,
plus paint which could be another $500-$800.
I hope that doesn't come as a surprise or bad news...
I don't think you can "repair" it to a point that would improve whats there. You might just live with it as opposed to mucking with it (which could make matters far worse).