Zoo Med Floating Betta Exercise Mirror is a small mirror suspended with a floating ball to help boredom from captivity. Put this in the bowl and watch your Betta perform natural behaviors; when he sees his reflection, he will exercise his fins! Zoo Med Floating Betta Exercise Mirror should be placed in with your Betta for maximum 5 minutes at a time, once or twice per day. Mirror's diameter: 1.80".
It is a floating mirror. My betta would rather do his little dance on the sides of the tank than in front of this mirror. I have only tried a couple of times, so it may work eventually. And it may work for a different betta.