My New 72"x20"x20" Tank Build

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I wouldn't scrap the entire idea just because you are having a difficult time finding a 2" Gate Valve.

Here's an unconventional idea that could help you test the concept. After testing, you might find that a 1.5" Gate valve might be fine.

Take your 2" drain and split it into 2 lines. Put a 1.5" gate valve on one of the lines. Make the 2nd line 1" for a first test. Rejoin the lines back into a single 2" and continue downward toward your sump and fuge. Put union fittings above and below the tees so you can replace it once you figured out how it behaves.

The idea is that the 1" line functions as some restriction, while the 1.5" gate valve controls the total amount of the flow.

Diagram attached.