We don't know how you did it, but you ruined your variant gas tank. Must be something in the gas. Did you get too crazy clamping down your petcock? Doesn't matter. We have you covered! New old stock variant gas tank. Comes as shown with no petcock and no gas cap.
Little tip- While you already have your frame torn apart to put this in, drill a few holes in the side of your frame. Wow! Fuel sight windows. Don't forget to slap your tank when you fill up! That extra burp might get you to the next gas station.
May or may not come in original Derbi plastic wrapping. If it does, don't let children or small animals play with it.