Is that the Aqueon Betta Falls tank?
They are really not appropriate for keeping bettas (not your fault, somehow companies are still getting away with teeny tanks for bettas).
A betta should have an absolute minimum of 2.5 gallons, and preferably 5g.
Per fish.
The Betta Falls tank is three separate compartments of around 0.5g each, and another gallon in the base for filtration.
So, yes, it's a 2.5g tank, but each fish has barely room to move.
It will be difficult to sustain water quality in this tank, and you Betta will be stressed and miserable.
If the tank alone doesn't perpetually cause fin rot, the stress will most likely cause him to keep nipping his own tail.