Right now I just have an in-tank filter (no overflow). I was just wondering how much flow I should have, and what size overflow pipe that would need.
Here's my basic idea:

I was planning on using a 10 gallon. I'm hoping that the large amount of substrate used as filter material will be not only easy to clean (just empty water and then scoop it out and rinse) but because of the large amount of material it won't need cleaning for long periods of time. Also, by re-using the material, bacterial colonies would remain intact.