Help with overflow

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Problem relates to this thread.

Drilled the hole for the 1" bulkhead (~20 minutes with dremel) and put the bulkhead and overflow in place and thought I would test before cutting into my plumbing.

Using a small submersible pump and measuring volume at 300gph found the bulkhead can't keep up. The layout how I planned is like the picture. If the drain pipe is submerged or even a short vertical piece at the end, I get pumping like in the video. If I just have the elbow at the end, the water in the tank backs up above the weir of the overflow.

Tried putting a Tee just after the bulkhead, just before the elbow, again the water level in tank backs up above the weir.

The refugium is a 20g and probably 300gph is a lot but really expected the bulkhead should be able to handle it. Any suggestions?

video