... I've done the same thing (Bio)3, it's a quick and easy way to make an overflow, cutting 1/4" slots or teeth effectively doubles the waterflow cross section over the weir which would increase it's pull on the surface film. Tim, I think we are saying the same thing. I said it increases surface skimming. I think you may have gotten hung up on the semantics of my previous post. Bottom line teeth do assist in surface skimming, and insufficient surface skimming can lead to film. That by no means says that without teeth you are guaranteed surface film. coast-to-coast overflows,most of which are toothless have been in this hobby for a while with no film problems. But I think we all agree, when done properly, teeth on a weir helps with surface skimming.