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.
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