20 sf bait tank ?

[Mikem8560]

My 1982 20' sf has a bait tank I. The rear floor. There was the remains if a airiator In there the kind that sprays water to make bubbles.
And on the transome above the hull drain there is a fitting with a plug in it.seen here

on the starboard side next to the bildge pump exit is a capped off through hull
I'm wondering how theese were set up was thus capped hole a drain hole?
Ever tank I've had you turned it on the tank filled and ocerlowed out a drain hole I cat see how this could work or take in water while running