James, since you are still in the planning stage, I thought I might mention a couple of things I learned about bottom drains, having installed one on our pond that was started back in the fall of 2010. I loved the idea of a bottom drain and reduced maintenance in getting the sediment off the bottom of the pond and the increased circulation of a BD with an air diffuser also made sense.
Bigger is not always better, I installed the koi toilet bottom drain with a 4" pipe, a 3" pipe would probably have been the better choice to get a better draw from the drain to the settling tank, from what I read after the fact.
My pond design has a waterfall at one end with a skimmer at the opposite end, the waterfall does a good job of 'pushing' the water towards the skimmer and the skimmer does a good job of 'pulling' it in, it actually works very well skimming the surface continually. Unfortunately when I turned on the bottom drain air diffuser which sits at the deepest end of the pond just in front of the skimmer it actually disrupts the surface flow from the waterfall to the skimmer completely. Even reducing the amount of air to the minimum still disrupted the flow to the skimmer. But I came up with a comprimise here, I installed a timer on the air pump for the diffuser, it comes on for 15 minutes and then remains off for 45 minutes. This ended up working well, the amount of circulation the bottom drain diffuser provides is really amazing as it circulates almost the entire pond towards the bottom drain. I noticed a difference right away with the increased sediment flow into the settling tank when the air diffuser runs, I would have hated to abandoned the air diffuser all together, the timer was a decent comprimise.
Well, just my two cents worth, I know I did tons of research online before we broke ground hoping to avoid problems and learn from others mistakes.
Good luck with the build.
