i agree with hayn. a Speedlight will give you better pics than a prime or macro lens. a fast prime is good if you dont have a speedlight, like the 50mm f/1.8. but image quality and depth of field at f/1.8 is not ideal for fish photography. i normally shoot at f/11-f/16, and you dont need a prime or a macro lens for this. that's why i say f/1.8 or other fast lenses arent needed.
a speedlight will be a better purchase. a good prime or macro lens may have a faster auto-focus systems and result in slightly sharper pics, but dont expect great results w/o a speedlight (actually, multiple speedlights), it just isnt gonna happen.