DD,
The 1800gph pump creating turbulence on the water surface will
probably be fine as long as you don't overload the pond with fish.
Oxygen transfer into the water primarily occurs at the surface. I
would probably hold off on getting an air pump and if I was very
worried about aeration would probably get another water pump to create
a water fountain or something that created more surface turbulence. I
just have a water pump outlet coming up through some bamboo to splash
the water and feel that it works fine, my goldfish are very active and
I have a lot of them in a pond around your size (I need to get the tape
measure out and figure out my volume).

James