I am more and more careful now with arowanas and plecos in the same tank. If an arowana is weak in any way, I think the pleco will creep up on it and suck away its slime coat.
Awhile ago I had a 12 inch silver jump out of the tank while I was out and I found him in a bucket nearby. I put her back in the tank and miraculously she came back to life and started swimming. However, she could never get strong again because I think the pleco sucked away her slime coat at night.
I also had a black aro that I am guessing was attacked by my Jardini that only went crazy and murderous at night when it was all dark. Since the black was weakened the pleco probably got to it too and eventually it died.
Fish keeping can be so sad sometimes.