whilst upping the shutter speed allows you to take "sharper" pictures. using the lens at its maximum aperture, say F2.8 is not the way to go. lenses, especially the type fitted to none SLR cameras, though they suffer too, would we working at about their worst at maximum setting. this is why lenses with a very fast maximum aperture are so expensive, it takes vast amounts of money to, try, and maintain descent quality when the lens is wide open.
if you can add light to you tank, even just temporary ones, and let the camera sort things out. that is unless you are experienced in manual photography. Flash is fine, though it needs extensive use of a photo editor to make it look real, and it brings even more problems with flashback. if you have no way of eliminating ambient light. try throwing an old White bed sheet over the tank, make sure it covers the tank so no outside light get in, directly. now cut a hole big enough for just your camera to fit through. this will give you a reflection free environment to work in. any ambient light that get through, will be very diffused and should not cause any problems. these are called light tents, and are used professionally to photograph highly reflective objects. if you have an SLR you can arrange flash heads on the outside of a tent, allowing you the benefits of ambient light, with out the problems of reflections flash back and high contrast suffered when using flashes.