I bought my betta about two weeks ago in an area that shared the same water with two other bettas, and one was sick. I bought him anyway because he was a good price and a beautiful crown tail so I figured I'd take the risk. Other than having odd behavior he seemed healthy. His odd behavior is hiding his head in the gravel and spending a lot of time underneath what I bought for him to hide in so he's completely in the dark. The past two days he wouldn't come up for dinner but I wasn't too worried because he was anxious to eat breakfast.
Now he seems really lethargic, laying sideways on the leaf that sits under the water for them to lay on. But he'll also dart around like he's fine. He doesn't look young so I don't know if he's just lethargic from age or if he has an illness I'm not aware of.