What do you do for an old betta?

Well a good start is making sure his/her tank is clean and doesn't have lots of leftovers at the bottom of the tank (this will prevent it from getting sick), Feed it a healthy and rather popular food (if the betta doesn't like the food he will spit it out or not eat), keep the Betta in a tank by himself (this will lower stress) and a very important thing to do is make sure there is enough in his tank to keep him busy/happy.