fish tanks are really low light - you will battle to get anything decent without a flash unit, unless you're taking a picture of a pleco sitting there on the bottom and your camera is on a tripod...
I bought a Sigma 105 f2.8 macro and it does not quite live up to my aquatic photography expectations. One needs a lot of light for this and on-camera flash and built in flash units are not the way to go either. One needs remote flashes to get the required results UNLESS of copurse the target is stationary - like a plant). My Canon A60 is still my better option for taking photos of my aquariums. However, I have rekindled my interest in photography and the macro is real handy considering I like abstracts and creative work. I am also still very old-school. I do not believe in making photos look good through using Photoshop - how I shoot it is how it looks. (Besides a bit of brightness/contract tweaking). Thanks for all the advice.
I also use a sigma 105mm f2.8 on my Canon film camera (I don't [yet] own a digital camera) - with any totally stationary target, I'm happy using natural light with it mounted on a tripod, but exposures are often around 30 seconds or so (I tend to use fairly small apertures, f16 or smaller, for increased DOF). These days, I tend to use an off-camera flash on a sync cord, even for very still subjects - it's much easier, not to mention quicker! A few years ago, I messed around with getting a slave flash to fire off the on-camera flash in a very large (~3,500l) tank with very thick glass - this worked fairly well, but required a lot of tweaking of the on-camera exposure settings to get it to work. I was actually using an underwater strobe light (Ikelike AQ/S), and firing it through the main glass, although I could theoretically have dumped the thing into the tank! Some serious juggling going on... Why oh why do we not have 3 arms?
rat-tail, try playing around with the shadow/highlight filter in Photoshop. You can rescue some pretty diabolical photos using that!