Hello. I am new here so first off - Hello to all. I am a gardening nut.

I am building a cement fish pond that will have an assortment of fish. Koi, Talapia, Barra, and a few others. The bog garden part will be a shelf, 30 cm deep running around the circumfrance of the pond. In most places it will be from 30 cm wide to 1 meter. At one end of the pond it will be 4 meters wide. The bog will be cement and filled with gravel. Water will be piped from pond to the back side of bog and flow up threw and over gravel back into the pond. The overall size of the pond is approximately 17 meters X 11 meters.

My question is: What is best,more effecient - to let water flow threw and over top of the gravel or to make gravel deep enough so water can flow only threw the gravel? I like the looks of the water flowing over top of the gravel and it also makes the pond appear larger. However, would it stil be effecient enough to keep water clear and clean? Thank you.