Besides using an airstone, have the pump bring the water into the bottom of
the deepest part of the veg filter, and rain from near the top of the other
end. This way it helps ensure there is a good bottom to top waterflow.
Otherwise the water at the bottom of the filter tends to become stagnant.

Well, I had intended to have the water go from the top to drain out
the bottom....would this prevent anerobic problems?