Puffing at his shadow

[sevenyearnight]

I'm not talking about the artificial pseudo mirror you see at an angle, I know they don't see that.
I'm taking about when a shiny background is attached to the back, a ghostly reflection can be seen. It's not as clear as an actual mirror, but my puffer and my betta could see their reflection when a background was added, and they were trying to fight it. I removed the background, and they stopped.

Again, I'm not taking about when you look in the tank at an angle and see a false mirror on the adjacent glass. I would hope that you would know I'm not that dense. The puffer could only see that if he were outside the tank looking in from my perspective.