OH no.
I'm so very sorry to hear this.
He probably was very, very ill - even moreso than any of us would have ever known. At least you gave him a very comfortable and peaceful place to pass on. I'm sure he appreciated that.
It could possibly be that the water in your area is either really hard, really soft or something might just be in the water. Based on everything you've posted here, you haven't done anything wrong at all.
One thing you could do is take a small sample of your water to the lfs and see if they can test it for you. That might shed some light on what happened. Also do know that in cases like this where their tails are pretty much gone, unless it's from some freak accident and they do survive, but rather from finrot or disease, it can go either way with them surviving.
Again, I'm really, really sorry.
It's pretty devistating to lose the little guys.