I am sorry for your loss, and sorry I did not see this post sooner. Dang holiday stuff occupying brain.
If your Betta was bought at adult size, its hard to tell exactly how old he was. It looks like to me fatty liver disease. This is a common ailment in older Betta, and since there normal lifespan is around three years, I would suspect this is the case.
I will assure you, it has nothing to do with the care and maintenance of your Betta's tank, but can happen from overeating as they get older. In the future, with new Betta, to prevent this, make sure to have one day a week of fasting, so their entire digestive tract clears out. Some suggest giving a shelled cooked pea the day before the fasting day to help clear out their systems, but I am not 100 percent convinced that is necessary.
Also keep a close eye on the ingredients of Betta pellets you use. Some cheaper ones use oils in them (canola), which can lead to this problem over time.
I am very sorry for your loss, I know how attached one can get to these little welcome wagons.
P.S. If it is any consolation to you, some Betta simply develop this with age, regardless of feeding schedule or careful food choices. Once a Betta has this, there is nothing you can do. Sometimes its just the natural course. You did well by him, and it is not your fault.
Last edited by Tiari; 11-23-2012 at 12:43 PM.
2 10 gallon tanks, 1 20 gallon tank, 1 Fluval Edge, 1 29 gallon tank, and one backyard pond.