I would say he's not getting enough food. A 12inch gibbiceps I used to have would lay upside down at the surface while you put koi sticks in his mouth

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Im no fish expert but when we were kids my mum had a tropic set up.. and we kept losing fish.. she said she caught him in the act one night getting a little tetra..

Also I had a syndontis just like the pic and cause he got so big, my sister took him on.. She has a large tank with a few fish... and a plec.. The plec does bully the syndontis in there.. and she feeds him wafers and cucumber and other foods..

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