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.
Now the first lens you mentioned is an L series, and the second one is not. I am not sure if you just left off the L for the second one, or if it isn't an L series. I believe the L series for Canon is the their top of the line lenses, Grade A professional stuff. I don't think some one who doesn't own a camera would be able to a huge difference between the two. I am not sure if I could, and I have been in the hobby for almost 3 years... haha
Anyways, good luck with your purchase, and looking forward to seeing some stuff on here.
PS: I am sure a Canon guy can chime in correct any information I messed up.