They can probably see reflections of themselves on the glass. Might not be a bad thing since they can't harm themselves by doing this, but in general with multiple clowns in the same tank, and with multiple species, be prepared for a lot of aggression, particularly coming from those maroon clowns!
Once that tank has anemones in it for the clownfish to host, I expect the aggression to subside. What are your plans for the tank? I see that you're in Sac. Have you seen any nice nems at Splash Aquarium? Every time I'm in the area I pay them a visit, and most of the time they have some nice ones.