Silver Arowana help

Hey guys;
I've been gone for 8 years ... Finally, got back in the hobby because my little son loves fish. Anyway, I have 3 small silvers in a 60gallon tank. And the biggest one out of the 3; stops swiming and sinks to the bottom. And I would think this guy would float already and die but just when I think he'll die. He'll start swiming and act like nothing ever happen.
I've never experienced this. What do you guys think its happening???