Well for Canon, the IS means Image Stabilization, so it will correct some errors for you if your hand shakes from trying to hold for a long exposure. That being said, most aquarium shots, you are going to want/ need to use a tripod. Since you will be using a tripod, IS isn't necessary, and in fact, most people say to turn IS off (VR if you are a Nikon guy like me ) when using a tripod.
You're fooling yourself. All of those shots would have been better with a tripod or some sort of camera support.