Easy solution, would be the one that Fish (chad) promotes.
Basically you add a ball or gate valve to your primary overflow pipe. Add a stand pipe to the second, so that it will act as an emergency overflow (if something blocks the first, rather than overflowing, it'll enter the second before overflowing the tank) Then you adjust the ball/gate valve so that theres a few inches of water in the overflow - keeping air from entering the primary drain and that level remains constant - requires a little fiddling, but it works great.
Andy
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