What a miserable life he would've had if he needed to have his chest drained so often. Not only that but the masses around his heart doubled his chances of having a painful time. You did the last act of kindness for him that you could and that's what being a pet owner means, doing what needs to be done, not letting our beloved pets suffer just to keep them around for our own selfish reasons. Wally knows that you loved him and he's at peace now. No pain, no suffering, no difficulty breathing. Soon peace of mind will come to you, too.