For a 20' long pond, you will need two drains, each going to their own settling tank or vortex tank. By all means....get aerated drains. There is a brand on the market now called "Koi Toilets". They are very nice drains for the money!! The purpose of aeration on the drains is not just to provide air to the pond. The air column rising from the bottom drain dome creates all sorts of wonderful currents that draw water and debris towards the drain.
Run 4" PVC pipe to the SC or vortex. Or let me stop here and make another suggestion. Go to this link and check out the static and fluidized kaldnes filter I made. It works fantastic!! I have one one of these filtering half the water in my 9000 gallon pond.
New Static And Fluid Kaldnes Filter (diy) - KoiShack
The water will gravity flow from your drain thru the 4" plumbing into the vortex. Put a valve in the line near the vortex to you can stop the flow for filter cleaning. If you don't understand how gravity flow works, basically the filters are positioned in such a way that the water level in the pond is the same as the water level in the filters. Since water seeks it's own level, and the pond is connected to the filter via a 4" pipe, the water level will be the same in both. As the filtered water is pumped out of the last chamber of your filter and back to the pond, the forces of gravity refill the vortex thru the 4" pipe.