'F/2.8' is the aperture/f-stop. Smaller numbers (like 2.8) mean that the lens can accept more light. But, they also make it harder to focus on close objects (due to smaller depth of field). The aperture can be changed on all lens, usually to values around f/22 or 'smaller'. The 'f' is the ratio of the focal length of the lens to the diameter of the lens opening which lets light onto the sensor. 'f/2.8' would be typical for a macro lens.
'100mm' is the focal length the lens: the distance that light travels from the front lens until it is in focus. Bigger numbers indicate greater magnification. BUT, neither of the lens mentioned by Titusville are zooms. For a zoom, you have to be able to change the focal length. The manufacturer indicates this potential by giving two numbers (e.g. 18-105mm) which show the shortest and longest focal length you can get. So, when you ask if the 180 will 'zoom in more', the answer is 'no': it doesn't zoom. But, it will give you higher magnification than the 100mm. This is probably more important if you plan to use the lens for telephoto work (e.g. birding).
To take macro pictures, you need to worry about how close the lens can focus to the subject. To get a true macro image, you need to be able to focus very close (5-6" or less). This means that the image on the sensor is about the same size as the object in real life (1:1). Most non-macro lens can't reach that goal; the image on the sensor is usually 1/6 the size of real life (1:6).
Macro lenses are also designed with optical systems which provide extreme clarity in their images. That lets you reveal very precise details on your subject.
A 100mm (or even a 60mm or 85mm) should be fine for tank photos. If you want to take macros of insects or wild life, a longer focal length (150-180mm) woudl be useful since you don't need to get a close and thus won't spook the subject (or get attacked

).
I don't know Canon but, in addition to the suggestions from Titusville, there are excellent macro lens in the 100mm range made by Tamron, Tokina and Sigma. They all make versions which would work with a canon camera.