Where should I buy a black ghost knife fish?

Strange. I bought a little baby one from Petsmart several months ago and now it's 4 inches long and very happy and healthy. Did you float the bag? Because that can do it. But your water levels are most likely off. They can be deadly without killing your old fish because thy slowly built up an ammunity to it but introducing a new fish can kill it because it's not ammune. It's happened to me before and it killed a Bala. So get your water tested and see if that's the problem