When I was at University of Missouri this summer for research work, a biologist who dedicated his life to paddlefish took me down to the basement and showed me his indoor pool filled with juvenile paddlefish. I fed them and they even let me pet them. The reason behind his research was that their neurons are actually extremely similar to ours in that they are very highly developed evolutionarily for fish, and apparently we can learn a lot from them. He was telling me that they get difficult to keep alive after about two years and almost 3 feet in length.