I should have mentioned that my dividers were not siliconed into the tank because I wanted them to be removable for cleaning or rearranging the tank. That's why there was a slight gap between the glass and the divider if the fish pushed hard enough on it. I didn't think they'd be able to move it, but I came home one day to find that was an incorrect assumption!
I went to the craft store again today to find a way to remedy my bettas being able to see each other through the black #7 mesh. The only thing I could find with smaller mesh was #14 Cross-Stitch Fabric at Michael's. I got the type made out of vinyl. It doesn't say non-toxic, but it does say that it's mashine-washable and it looks like it should be okay. I plan to just stitch a piece over the black #7 mesh I'm already using. The only downside is that it's white. :(
This vinyl cross-stitch fabric came in a roll. Has anyone ever used it? Is it fish-safe?