I'm stumped. Overflow lines are usually 3/8" and are welded to the top of the tank with a braided line going to the overflow plumbing. Seems like you are getting a siphoning action as once it starts to overflow, it keeps siphoning out the tank. But, I've never heard of an overflow having a drop down tube in the tank to make that happen. So that's not the issue. I know that some tanks swell when filling because the overflow line is clogged partially by bug nests and can cause issues filling as well as popping the overflow line off the tank. If the fill volume is faster than the air escaping from the vents, it can also swell the tank causing water to blast out of the fill or overflow as the tank comes back to it's normal shape....hmmm.
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