Once you put a few goldfish in a pond that size, you will have a few hundred goldfish after the first summer. How do you plan to control the goldfish population?

Schmily,
The question is the bottom drain going to have water move through it by a pump or by gravity to a settling chamber? Why is this important? If the water is moving by gravity to a SC the only force on it is the difference in the height of the pond and the height of the water in the SC as water is being removed minus the friction of the water moving through the piping to the SC. The difference is the head pressure of the system. This head pressure is normally any where from a .5 to 3 inches. As an example if you have 4" piping going to the SC and the total length of the pipe is 20 ft including the friction loss of fitting expressed as length of pipe and the drop in the SC is 3 inches then we have a flow through that drain of 8162 gph. Which is a lot of water but with a pump it could easily move over 20,000 gph through the same piping. If we change the difference from 3" to .5" then the flow would be 3,146 gph but amount by pump would stay the same. So why is this information important? The amount of water moving through the drain and the design of the drain will determine how far from the drain will be able to suck in debri. So what does this have to do with your question? As can be seen by my examples the pump can easily have a greater distance that the drain remove debri. So to increase the flow of water around the drain on a gravity system or low volume pump system someone came up with the idea of an aeration drain. The column of air coming up from the drain can greatly increase the flow of water around the drain. It does not increase the flow of water through the drain but if designed correctly could increase the area the drain can clear. If you have a pump system there is no down side unless you start to suck air into the drain. But on a gravity system to a SC the point is to move the fish poop to the SC where it can be removed as a solid. This waste is very fragile and about the same weight as the water. What usually happens with aeration drains is that most of this waste is lifted up into the water column above the drain and then is broken up by the water. So almost always you are better off putting more drains in without aeration than you are by putting in an aeration drain.
Mike

Thanks Mike - that makes sense. Is there a formula to determine how much area a bottom drain can clear? I'm planning 2 levels of the pond with a small waterfall in between so I guess I would need at least one drain in each level.

Sorry there is no formula for that as there is too many things that will affect the area that the drain can work in. The rule of thumb for a gravity drain is a circle with a 10 ft diameter. But this is just a rule of thumb and it might be low for you or it might be high. It is the same for a pump system if the flow rates are the same but if the flow rate is increased then the diameter is increased.
Mike

Sorry to bring up an old thread but I'm wondering the same and I am also going to have 2 levels separated by a little spillway of sorts and waterfall on the higher level... Would I be better off with a drain on each level? And can I y them together to go into the inlet together? I was thinking 2-3" drains.. Let me know what you think