Not sure of the exact size, but it's 13x13x, 1.5-3 feet deep.

Okay, I'll assume an average depth of 2.25 feet. Your pond is
13 x 13 x 2.25 x 7.48 = 3,160 gallons.

The spitter is a 2-3 foot tall gargoyle, ...

Hrm. A "spitter" is your only water flow? That doesn't sound like
it could be very much...

You should move about 3,000 gallons of water per hour. I'd look
into a Sequence 750. Lots of gallons moved, low watts consumed.

This is the first time I've ever seen the fish all together doing the gaping
thing.. maybe it's some common group behaviour as well?

Almost always means not enough oxygen in the water.

C//