about the same thing happened to me, but it was with a Clown Knife. He was hanging out near the top of the aquarium and not hiding like they usually do (which was bad sign). I didn't think anything of it though and I purchased him and tried to feed him frozen brine shrimp, he didn't eat. 25 hours later, hes dead in an upside down U shape.
I believe now that he was stressed before I even bought him. (possibly from getting shipped to the fish store and having to acclimate to that water and then acclimate to my own water so very fast, and I bet they didn't even feed it before it was shipped and arrived at the fish store which contributed to the stress of the clown knife and made things much worse)
& luckily the owner of the fish store was nice and gave me another clown knife for nothing.
Lesson I learned: Put the knife fish on hold that you are wanting to purchase, wait a day or two, then return to the fish store and if hes still alive, then you know that he should be ok. If he dies the day or two after you put him on hold, then you know he wasn't the one. & then you shouldn't have to deal with a fish dying so fast after purchasing.
Also, I've heard when knife fish are that young, they are much more susceptible to water conditions and changes.