Gareth is getting worse

[Cloelia]

I haven't seen him displaying any of the signs of external parasites (but I wish that's what it was so I could do something about it).

I suppose it could just be an injury. That would explain why it showed up overnight, but he'd been acting strange before that, too. I guess it could be unrelated.

I did notice that he's more active today than he had been for the last few days, so maybe he's getting better.

[Oldfishlady]

Since you are already doing 100% daily water changes, I would stay with that and maybe add 1tsp/gal of aquarium salt along with the 100% daily water changes to help prevent a secondary infection, your temp is good at 75F to help keep bacteria down.

However, as the Betta gets older they do tend to be more sensitive to cooler water IME and can be symptomatic (stop eating, lethargic, hiding, stress and illness) you may want to think about a heater for him once he recovers and keep his water temp in the 78-80F area

[Cloelia]

Thanks for the advice!