First you should decide what you want out of this camera. I think many, many people by into the Rebel line who would have been more than happy with an advanced Powershot. I suggest you look into the G9. It is a great little camera and should take great shots of your tank. This camera is powerful, inexpensive, and practical.
If you want to really step up your photography beyond the realm of reef tanks with an Xsi, don't forget all the lenses the camera will be useless without. If you want to do it right, I would suggest a 17-55 f/2.8 IS, 100 f/2.8, and if you want zoom a 70-200 f/4 or f/2.8 (IS). The 17-55 and 100 Macro would be great for reefing, the 70-200 is likely a non-reefing lens although it has its conveniences. With a 75 gallon display, I don't see it getting much tank playtime though.
There are many lenses, tripods, flashes, and other equipment that all do specific, often necessary jobs. Tell us what you want this camera to do, and we can steer you in the right direction.