What species of knifefish and what size is he? What size tank is he in and how much filtration?
Bettas are by nature solitary, so would do better alone. In a tank around 5gallons with a trickle filter (they dont like or want fast filtration) and a heater set to around 78-80degrees fahrenheit.
A few plants live is preferrable but soft fake ones will do just as well.
Oh and I forgot to asnwer your question, no bettas arent tough, they are very aggressive towards fish of their own species and sometimes to fish that fit the description of their own species, but 9 times in 10 its the betta that will be victimised in a community tank becasue of its large fins and being so slow moving.
