I highly recommend getting a speed light that sits on the top of some cameras (DSLR, SLR) that you can bounce off a reflector next to your aquarium. unfortunately they're kinda expensive :( I rent them from my school because I don't want to buy one yet because of the price. You can also use a flood light (very cheap at home improvement stores) and point it through the side of a aquarium to illuminate your fish in a more natural looking way. Also this way the fish's eyes can adjust to the bright light.
Full spectrum lights are better for photography if you decide to use a floodlight, especially if they are 5000k or 5500k which is sunlight
I try not to use flash with my fish, only when I first got them for good sharp photos of them displaying. I imagine it as having a strobe light(the big lights in studios) going off in your face without a soft box, it just isn't fun and fish don't have eye lids to protect their eyes like we do.