I have a 300 gallon pond. The filter and pump is 1800gph which is way
overkill.

There are smaller pumps. For a pond your size, 150 to 300 GPH is
more like it. The MD2 is about $50.

http://www.aquaticeco.com/index.cfm/.../8804/cid/4016

If cost is an issue, you might compare the probable 180 WATTS to
the MD2's 25, and estimate how much money you'll be saving monthly.
Quite a lot, I expect. The pump will probably pay for itself in
about 2 months.

So cant I just add a few air stones or air bubbler when the
pump is off for about 1-2 hours?

Of course you can.

C//