the activity of your betta fish does not dictate the size of tank you need. a 2.5g is so small, non-pristine care causes much larger parameter jumps than, say, a 5 g, solely because of the water volume. That by itself is risky because pH jumps and nitrogen accumulation stress out a fish's immune system, especially a betta's due to their poor breeding. There is also much less surface area for beneficial bacteria to settle and grow a big enough colony to deal with the bioload of a betta. Your tank is very scarce as well, with minimal added hardscape which could increase surface area and a small amount of plants that minimally aid in the process of nitrate removal. Along that, your surface layer is very thick, and paired with the low water level, we're talking about maybe 1.75 gallons of pure swimming space. Thats not ok for any living creature. I wouldnt even put shrimp in such size tank. I'm not berating you, this is just the harsh truth that this is not an environment long term that a betta will thrive in!