Ok. You can be very persuasive. Neither lucky, nor hard work but mastery!
It is actually very simple,atleast after you take that first shot and learn how to replicate.
I have some ground rules, I mostly shoot the fish I have been keeping, and which are well accustomed to my presence.
That way they don't hide, the moment I bring out the camera, or they don't stop behaving normally, if I reduced the level of the water to take some shots.
Eventually you know how, each one if them reacts and you can predict their next move in the next instant and there you get the shots you want.