Rory you won't regret the XT, in fact you may never feel the need to upgrade...
There are many ways to do Macros depending on what you're shooting. I really try to shoot with available light as you get much more natural looking shots. That being said, even with the amount of light we shower our tanks with, it pales in comparison to shooting outside, even on an overcast day. Your camera's metering system will quickly make that apparent. Using a flash pointed directly into the tank will give you reflections as well when using a flash so what I do when using flash is to set my flashes up at 45 or so degree angles to the tank glass to eliminate glare. You will need to compensate for the awful colour of a flash with your white balance settings, or shoot in RAW mode so you can post process this later on. If I am shooting a coral, I tend to set up on a tripod, frame the shot and use a remote shutter release, using as small an aperature as I can along with the fastest shutter speed possible. Up to 800 ISO gives me great results, even when blowing images up to huge sizes. (I have a printing business so I can make prints up to 4 feet wide - original image quality is important and has been of no concern using either my present 20D or my previous 6.3MP Rebel). When shooting fish or any other moving target, I ixnay the tripod, hunt and chase them along the front of the glass and shoot lots of shots hoping to get "the one".... this is the biggest advantage digital has over film IMO. As for distance, it really depends on your lens and composition you are looking for. You CAN lean right up against the glass as you are focusing beyond it anyways. Extension tubes work better with primes and are pretty much useless at focal lengths less than 50mm. You CAN use them on some zooms, but usually only at it's maximum focal length.
There is no ONE way to do Macro work... it all depends on lighting & subject along with the desired depth of field and working distance. The closer you are to your subject the more light you will need and less depth of field you will have. For example let's say you are shooting a coral with a 135mm lens and a working distance of 2 feet using an extension tube. Your normal tank lighting will allow you to shoot at 1/160 at ISO 800 and f4... Your depth of field might only be 1/4 of an inch, so if your coral is 2 inches from front to back and you focus near the front, only 1/8 of the coral will be in focus... if you want more depth of field you will have to add more light, stop down the lens, and/or increase the working distance between the film plane and the subject. You may need to use a lens with a longer focal length and increase the working distance if adding more light and/or stopping down isn't an option... sorry if I'm going in circles here... just trying to point out that there is more than one way to skin a cat when it comes to taking pics... the beauty of digital is being able to see right away if you got the shot or not...
If I could only have 3 lenses on a budget... hmmm....
1). The 17-85 is a great economical beginning... gives you a decent wide angle, and probably the best "kit" lens Canon makes. IS is a bonus as well...
2). The 70-200 4.0L unless you can afford the 70-200L or the 70-200L IS. These 3 lenses are the most popular and highly regarded in Canon's line.
3). Any prime 2.8 or faster between 50 and 150mm. Once you see the sharpness and contrast of a prime - not even an expensive prime, you won't use your zooms as much. "L" quality primes are even better yet.
Once you have those, don't bother buying another lens unless it's a 2.8 or faster... the exception being the 300 4.0 IS... another GRAND... lol.