What is the temperature of the tank? Betta's like it warm. My tank stays at 80 degrees...I only have the betta in there though, so I'm not sure if your other fish will be comfortable at that temp.
Do a bit of research online on Bettas and look up any symptoms your's may have (such as fins clamped close to the body; not spread out). There's a good bit of info out there. It's always helped steer me in the right direction.
Post a pic of him too, in case he's showing any symptoms someone on this site may be familiar with.
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