I think that under certain external lighting conditions, and in just the right spot, they can probably see a bit of a reflection off the glass in the corner. It won't be a solid mirror that we see, but I bet they can see enough colour and shape to trigger their inner demon.

I've had two long term tangs in my reefing career, and both have attacked the glass in the same spot in the corner, and only when the glass is really clean.

I bet if you were to put black paper over one of the sides, it would probably stop.

I have a bangaii cardinal that does this when the glass is freshly cleaned. If there is a buildup of film, then he doesn't.

So I think to some degree they see something. What that something is, is anyone's guess. Maybe a slight reflection like when you look in your rear view mirror in night mode? You see your back seat but you also see a bit of the road behind you...

Anyways, if it was fully reflective, they wouldn't see you coming and swim up to see you... :biggrin:

My wrasse and clown fish will do the same.....they are very territorial and will attack towards the glass.....but its during the transition from light out in the house to dark....and maybe the lights are changing at that time as well (not sure of that part).....I just assumed the glass at some times must be reflective to them

.....or I have some fish that need anger managment classes!! LOL.

Same thing with my foxface.