Don't they get 36" in the wild? I have only seen some around 24" at the Denver aquarium years back.
The claim they reach 36 inches is untrue, this size was copied from an old
Polypterus care sheet and, as like a lot of online care websites, they umbrella Polypteridae together (you'll note, if you trace back the references of this size, it leads to a dead end). From memory, most of these care sheets are based from both or either
Polypterus endlicherii and
P. senegalus as these appeared earlier in the trade for many countries.
Ropefish tend to reach
around 18 inches during their long lifespan (even in the wild), infact, I'm not aware of any larger than this, although I don't doubt there are some. 24 inch sounds rather too large, although 20 inches, maybe 22 is probably more reasonable. A large public aquarium provides a safer enviroment than the wild, and quite often better care than what is possible in the aquarium hobby, so specimens can reach sizes which wouldn't normally be possible. Aquariums and zoos really do hold some hidden gems (I'm sure Denver Aquarium is no exception), although, due to their highly serpentine shape, they easily look longer than what they actually are.
Hope this helps!