"Mike \(mixmanx at AV and DVD Forums\)" writes:

I've just finished building a small pond using a liner, but I'm not sure if
it's a good idea to put gravel in the bottom or not. Being a tropical fish
keeper, it seems the natural thing to do, and I think it will look better,
but will it be a pain to keep clean?

I use black epoxy coated aquarium gravel in my pond. The koi love to root
in it and it has no algae or debris collecting -- the koi just mouth
it too much.

I didn't want regular colored aquarium gravel as I think koi look
better on a dark background.

I can see where bigger gravel or stones would be problematic.