Luananeko
Aquarium Advice Addict
What temp do you have the isolation tank at? I find my bettas tend to be pretty sluggish if I keep them in the 76-77 range like you said your community was at. They really do much better if you bump it to the 78-80 range, but then you're running into the problem that your danios prefer it on the cooler side... I don't think that's the only thing that's going on here obviously, but it may be a factor.
If your water parameters in the community are all looking good other than the temp then the only other thing I can figure is that maybe he's one of the boys that gets stressed at the activity level of the tank. Not all bettas do well in communities, and not just because of aggression/harassment issues. Sometimes you'll have a betta that gets nervous from the community's more active members zipping around constantly, even if they're not bothering him directly. The constant stress of seeing all the hustle and bustle combined with the cold temp could be getting to him. If that was the case I'd expect him to bounce back a bit in the isolation tank though (even with the cooler temp), which it doesn't sound like he is...
How's his eating and what are you feeding him? Maybe he's constipated?
If your water parameters in the community are all looking good other than the temp then the only other thing I can figure is that maybe he's one of the boys that gets stressed at the activity level of the tank. Not all bettas do well in communities, and not just because of aggression/harassment issues. Sometimes you'll have a betta that gets nervous from the community's more active members zipping around constantly, even if they're not bothering him directly. The constant stress of seeing all the hustle and bustle combined with the cold temp could be getting to him. If that was the case I'd expect him to bounce back a bit in the isolation tank though (even with the cooler temp), which it doesn't sound like he is...
How's his eating and what are you feeding him? Maybe he's constipated?