They're cute buggers when little.... Then they turn into Ball Park Frank sized fish torpedoing around the tank. They are entertaining though.. I've got 2 in my 55g. About 3yrs old.
A lot of activity I'm sure! The most I had at once was 3, and that's plenty for a measly 55g. They were super tiny when I bought them. One suffered from bloat when I had them about 2yrs. I couldn't get him to recover and he died. My remaining two are doing fine. They're always dancing belly to the glass when they see you. They are highly prolific eaters, which can be a bad thing with mixed stock. They'll eat most of what hits the water.