answer:How old is your betta? Or how long have you had him? As a tropical fish, bettas are not very long-lived. The ones I’ve had usually live for about 2 to 3 years. When you buy them at the petshop, they are most likely already half-a-year to a year old. So if you’ve had your betta for more than a year, he’s probably suffering from old age. The other likely possibility is that the recent illness he had took its toll on his overall health, and not having eaten for awhile depleted a good deal of his energy and body mass. He might be carrying an internal bacterial infection (as he also had fin rot). One more possibility is that your betta is suffering from an air bladder problem, due to aging, and it disables him from swimming properly or even just moving around normally, so all he can do is lie at the bottom of the tank.