A full grown, 20 inch knife needs
a tank with 2feet of width and 2 feet of height. Length is up for dispute, anything from 4-6 feet sounds good. But that puts it at 120-180 gallons.
If they make 4x2x2 aquariums that could work. I don't know still seems crowded. But then the fish isn't that active. But it's nocturnal so maybe it'd be active at night.
This is why tank size isn't a solid fact.
I had a dream that I got a 6x2x2 tank and got a ghost knife (thanks Sakura). Along with angel fish and a school of pacu. PACU!