HELP, my fish's lives are at stake

I have started ym tank recently, and all my life my father has had tanks, so it is now my turn. I wanted somthing more exotic, or somthing new, somthing we've never had. My mom picked out neons, and green fire tetras, and my brother picked a powder blue gourami and we all decided on a red tail shark and a glass cat. It was now my turn to pick out a fish, i go to the store and i see an african knifefish. Now i was smart, i know about compatibility for the most part, and i asked about the knifefish and i was told it is a perfect community fish, i said yay and ill take it. so i got it home. 2 days later, im missing 3 neons and i fear more to go. I dont want to take the knifefish back because the owner i know and trust but it was his assistant that gave me advice. how do i save my fish and not return ginsu (the fish's name) please help me, how do i feed him to keep him from eating my other fishes.
-Colin

Thanks

I tried to take it back and said how cool it was but i had to cause it was killing my other fisha nd the guy gave me his old old old tank and i set it up and ginsu is in it surviving well and im going to start a african cichlid tank
yay
win win

is it ok if i feed him earthworms?

yay, i will feed him many of that cause i have a lot of them earthworms

yea he is in a tank w/ a big goldfish bigger than him and a huge agressive redtail shark that i had to sepperate too. i found my knifefish LOVES flys cause one flew into the tank (i accidently left the cover up) and it was gone in 2 seconds, so i fed him another