Actually if the fish is new you see a lot of skittishness and shy behavior.Thanks, Jibran!
But fish must be affected / scared by the powerful overhead flash, no? I mean until the first flash and during the instant of the first shot they'd behave as you say ~ naturally. Do they continue afterward?
I love the pitch black background and the brightness of the objects being photographed.
But once they are accustomed even if the flash is hit, you don't see skittishness, they get wary of the flash. After the first few times, they just are unfazed of the flash, and act as if nothing happened. If you look at the above pic, it is a shot of the 56 I shot that day, tors are skittish and fast yet, in this case they are swimming on the top of the surface.
I Never shoot new fish, the archers I did not shoot for a whole year, as they are a skittish lot, and I waited for them to get a bit tame, so that they don't over react to the flash and hurt themselves.