My dragon pipefish died last night

I have no idea why. We got him 2 weeks ago and he was doing great. He was eating well (live food only, which there is still lots of) he coloured up really nicely 2 days after adding him and my other pipefish is alive and well.
I just don't get how he could be doing so well then suddenly die. He was still fat and healthy looking (other then being pale) when I found him dead
There is only 3 explanations that I can think of: 1) The store I got him from has had a few of their fish (that other people have bought) die on them for no apparent reason 2) I know pipefish like low flow but maybe there isn't quite enough flow or 3) He was already old and it was just his time.
Maybe I'll never know, it just sucks loosing a fish, especially one that seemed to be doing so well.
