This is exactly why Betta fish NEED a minimum of 3 gallons that is filtered and heated to 77-82 degrees. On the left is a Betta I fell in love with at the pet store and came back 3 weeks later to find him still sitting on the shelf in his small cup. I bought him only to find out from an expert that he is a Half Moon Dragonscale. On the right is a picture of what a healthy HMDS is supposed to look like.
Prolonged exposure to “Betta cubes”, unfiltered water, and unheated water will do this to your Betta. Fin rot this severe is extremely painful for them. It would be as if someone shoved you in a bathtub full of bleach for the rest of your life. Bettas cannot replicate their natural habitats themselves and it is up to us as responsible owners to do it for them. If you wouldn’t lock a dog in a small filthy kennel for the rest of it’s life, so don’t do it to a fish. Please research your pets before putting them in unfit conditions, because letting their fins rot away in bowl stripped of their natural requirements is cruel and unnatural. We took them out of the wild, the least we can do is provide them accurate living.