Why did my African dwarf frog die?

I had bought 12 ADF from brookstone to put in ny 40 gal tank back in August of 2010. A few months after, two of them died without any apparent reason. Now I found one today belly up on the bottom dead. But he had a odd looking green area on his belly, about the circumference of a pencil eraser. What did he die of and why. There are no fish in this tank, it has a small filter and is kept clean with water changes. The water is in check perfect, and has a good ecosystem between bacteria and live plants. Someone told me not to buy the frogs from brookstone, only buy from a pet store because they are healthier. Is that true? But more importantly, what was that green on my ADF's belly?