I'm sorry for your loss.
I bought three rummynoses together, and I have one left. I am gathering from what I've read here from other people, and my own experience, that they are a very delicate species of fish. I have no idea what killed my first two, but each time I knew something was wrong when they decided to hang out on their own in my foliage instead of schooling. I was lucky(?) enough to come home to find them already gone and didn't have to watch any suffering and then euthanize. I've read that sticky too, though, and it is very helpful.
The thing is, I feel pretty helpless when I lose a fish; it's not like you can take it to a vet and say "It's just been sitting at the bottom" or "floating near the top." You have to try and figure out what the problem is and fix it, and that is sometimes impossible. I've read so many accounts on here of unexplained fishy deaths. It's hard.
But at least you did the right thing and ended the fish's suffering.