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it actually looks as though he is attaching to the glass. he is using the glass as one side of his tube, therefore he only needs to make one side. mine actually has done this, has lost its crow 2 times, and came out of its tube and rebuilt another one. ive had mine for 4 years.
he may also be getting attacked at night as the fish claim a territory.
i had some chromis' that attacked my sailfin at night but were peaceful with him in the light. he's probably fine and it might go away or might not.