I would suggested at least 75 gallon minimum. They aren't aggressive as such, but they are predators so they will eat any fish that will fit in there mouths. They are nocturnal, so they do like to have a cave to hide in during the day.
They will come out at feeding time, but they don't really like exposed areas or bright light. So if you have some sheltered areas in your tank - it's much more likely you will get to see the knife fish regular.