Bear in mind I'm not professional so I can only equate your situation with what I observed in mine. I first had him in a 3L (<1 Gallon) temporary home until my cousin brought the 20 gallon (I ended up having to buy a 10 gallon the next day cause issues... lol) In this temporary small 'tank' he just chilled on the bottom. Which had my mind racing that his finnage was so heavy he couldn't swim much without tiring out (another reason I couldn't use the 20g - cause it was deeper...) Anyway, I put him in the 10g after it reached the appropriate temperature. His lethargy was instantly cured - he swam around up and down didn't really sit still (so obviously his ball gown of a tail wasn't too heavy) - After my 2nd cleaning I added a bunch of decorations to the tank - probably about 50% of it was taken up by the decor/fake plants and etc. - and he began chillin' out more again. So knowing that my decor, heater, and filter took up a good 5g of tank space in a 10g tank - I'm going to guess that your apparatuses and decor are taking up close to 50% of your tank, and so your fish just doesn't have much room to really swim or explore. (After all, we have a completely different swim quality between a bath tub, above ground pool, and public pools - it's pretty much the same for fish
Check facebook market place for stands/tanks with stands. I saw a sweet coffee table aquarium I was interested in (if only I wasn't on the 2nd floor!) your boo probably just needs more space to just keep swimming
swimming swimming swiiimmminnnGGGG.
Only thing I really see concerning is you potentially over conditioned your water;
"What is the source of your water? store bought betta water because the well water is not good
What additives do you use other than conditioner? What brand of conditioner? don't have it on hand not home atm.
^ you used a conditioned water, and added a conditioner to it?