The older a betta gets the more important it is to keep the water parameters on the money. Keeping the tank clean is a must. I know when My Harry died He was 9 years old. He was one of the oldest betta I have ever had. I attribute the long life to the right water conditions, The best food, and the best care you could give a fish. I know right up until the very end he never showed any signs of being ill. The night before I lost him he was swimming around his tank as he always did. He ate well that night, and when I turned his light on the next morning he was in his night bed and floating. I would have to say he died from old age, and died in his sleep. 9 years for a betta is a very long time and you should be very happy for his life. The best you can do at this time is to keep an eye on him and keep the water crystal clear.