Macro is a setting on your camera. On Canon's it's the 'flower' setting. Your camera needs a macro setting if you're going to be taking pictures of items 6" or less from the lens, especially through aquarium glass.
When I use the macro setting, I'll ramp my camera's ISO up to 400, select macro, turn the flash off, and turn on the high-speed action setting. So when I take macro shots I'll be taking three quick shots w/ one push of the button, in less than a second. This high-speed shooting seems to work well, as it helps reduce blurs from unsteady hands, as the middle picture avoids that first shake from pressing the button.