I'm sorry to hear about this too, I just lost a female beta to that last week. Just think of this.
When I bought my last beta, it was at wallmart and all there beta's were in little cups. Now I know most stores do that, but these all had covers and no air holes. I tried to explain to the moron that you need to take the tops off so they could get air, but he didn't understand, fish breath water he said. I'm sure all those fish died within a few days, at least you gave yours a good life until the dropsy.