Never tried one for a pond that big! The rule of thumb is veggie
filter = 10% of the surface of the pond as a minimum. More is surely
better! As I understand it, you need at least a 45 minute flow-
through time to allow settling. You can always divert some of the
water around the veggie filter to slow it up. Our veggie filters are
18" deep with EDPM liners.

At about 30% the pool surface, you should be doing fine! The veggie
filter needs to have fast growing 'rooty' plants to grab nutrients,
convert them to growth, provide surface for bacteria to transform
nitrites to nitrates. You will know you have too many plants when
they stop growing.

Your veggie filter will be a fine baby tank, if you want. We run
mosquito fish to avoid mosquitos.

Other folks will probably have more scientific advice.

Jim