What do I replace this "plug" with

[jsridley]

New house, new pool. There's a designed hole between the spa and the main pool. Not sure why it's there as the spa spills over the pool when pump is running. Previous owner plugged it up, I'm assuming to help keep spa hot when using it, which makes sense. However, whatever he used is now a tetanus factory of rust and corrosion. I haven't tried removing it yet, but assume I'll be able to figure that out. Anyone have idea of what I should replace this with? Should I not replace it at all and allow water to flow between spa and pool?
Thanks for the advice.

Spa Side:


Pool Side:

 

[frustratedpoolmom]

Problem is without knowing if there's threads on the spa side... are there threads on the pool side? Can't tell from pic. Plugs are pretty much typical standards sizes, but if no threads you would probably have to use a rubber plug, but I'm just guessing... Have you tried doing a google image search, maybe you'll see something similar?

 

[kadavis]

Aladdin Pressure Test Plug 2 inch Pipe 800-11 - Hardware Plugs - Amazon.com