Ropefish could easily do a 55 gallon, for 2-3. Well, I guess most people say it's improper, but they aren't active, and they don't need to keep their bodies straight like knives. 55 gallons is good for a single bichir.. but again the width of the tank is the same as the length of the fish. All personal opinion with tank sizes once you start getting up there. I've heard everything from my tank is horribly overstocked to horribly understocked (if that's even possible). 75 gallons is a great tank size to start with these guys, that's basically what mine are living in, the 90 gallon is just 4" taller, doesn't really do much for these guys.