I've never had a tang do that, but I have a domino damsel inmy FOWLR that does the same thing. Usually only in the evening when the room is darker. I think it may be seeing its reflection in the glass and reacting to it with a show of aggression.
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I retired and got a fixed income but it's broke.
Ed
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50 gallon FOWLR, 10 gallon sump.
130 gallon reef, 20 gallon sump, 10 gallon refugium.
10 gallon quarantine.
60 gallon winter tank for pond fish.
300 gallon pond with waterfall.
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