Who Gives A Fish Forums

Also, let's talk about tankmates. Some people throw in aggressive fish or fin-nippers, then wonder why their betta's stressed. Do your research before adding anything else to the tank.

Yep, and don't forget about water changes. Some folks think they can skip 'em if the tank looks clean, but nah, that's a sketchy move. Ammonia builds up fast in those tiny setups.

I see you mentioned Lupin has a heater in his tank, so that's great. Considering his stress levels, I'd suggest looking into reducing the filter flow - bettas prefer calmer waters. Those daily partial water changes you're doing are a good start, though.

I'd say delta or halfmoon's a possibility. If you can manage a video, screenshot the best flared pose, might give a clearer idea.

I've seen it before, betta's changing colors isn't uncommon. He's eating, swimming fine, seems like you're doing a great job taking care of him, so I wouldn't worry about it too much.