Not only is this shop giving bad advice, as pointed out, but it is heavily over-charging you. No way should a Senegal Bichir. I have 3 and never paid over £5 for any of them. If I remember correct, I am sure they were only £2.50 each. Anyway, as Neale says, it is unlikely it took down the Knifefish fully alive. My guess is the Knife was already dying or dead and as more of an opportunist than out and out predator, the Bichir took advantage. I had a similar thing with one of mine and an Eel. Unfortunately the Bichir choked on the Eel too.