the reason it is uncommon is because they live in the wild which should be more prone to disease injury and predation and fishing. However to say a pike will reach 20" in an aquarium is in IMO ridiculuos. Most of your 20" pike are eaten by other large pike and Muskies and people like me who love them because they are one of the tastiest freshwater fish there is. Fish do not have to deal with these issues in our aquariums unless they are being kept irresponsibly. So recommending a fish that grows that large and is a voracious and EXTREMELY fast predator for a tank that size SUCKS. I would feel sorry for the fish if he ever tries to get up to 1/4 speed or tries to turn around once he reaches 24" probably 2-2.5 years. They are not extremely flexible. Not like a Silver Aro. I would LOVE to keep one but would not even attempt it unless I had a tank was at least 10'x4' minimum.