I guess I disagree with everyone.
I think a flash is a great tool! Especially for aquarium photography. You can never have too much light for photos. Using a flash will allow you to use faster shutter speeds and lower ISO giving you sharper images with less noise.
I'm unsure of the canon equivalent as I am a Nikon guy, but I recommend a flash that can be used remotely, has a manual mode, adjustable direction, and a light diffuser. It looks like the Canon Speedlite 430EX II would work for your needs but any flash that can be removed from the camera body will help balance out aquarium lighting. By removing the flash from the body you can help control harsh hilights and reflections. The use of a light diffuser will go along way in giving your subject a softer, more even, natural light. You can always buy a diffuser but I find using cloth, in addition to old milk containers, or a cool whip lid works just as well. It takes some practice and a whole lot of experimenting but that's the fun part.