I have mine mainly by accident - I bought a 220 litre aquarium off ebay with the intention of keeping 50 odd white cloud mountain minnows in it by the side of my bed. It was advertised as already having five fish in it, but from the pictures it looked like they were just platies. Fine. whatever.
What it actually came with was three platies (and some of their fry) two danios, and a four inch Leopard bushfish - none of which the owner told me, and I didn't know until I opened the buckets and did a little internet investigation. I evacuated the platies and Danios to my son's tank, bought some silver dollars to keep the leopard company, and started making plans to buy a bigger tank. I had some loose plans to acquire a 700l from ebay, put the leopard and the dollars in there and maybe also buy an African knife fish. I had read about the black ghost, but decided against it.
However, last Friday, I was investigating a fish shop near me - it had been open a couple of months and was operating out of a shipping container (not joking), and I spotted a 10 inch black ghost knife fish. He was attempting to hide behind the filter in a tank 12 inches deep and about 2 ft long - maybe 100 litres tops - and he was sharing the space with a large synodontis and a red snakehead.
I asked the owner about the black ghost, and was told he'd been there about a month and that he'd been eating flake food. So I bought him in the full knowledge that he would probably outgrow my tank fairly soon, because it was better than leaving him where he was.
aqadvisor isn't throwing up any red flags with their current sizes, but it's only a matter of time - especially as he's eating properly now - and I'm making concrete plans to get a 500l - 700l after Christmas.
I've put a video of them on youtube: