I love my Xti. 75-300 is a pretty killer zoom lens. Take it outside and play with it by looking down the street. Take it to a ball game, it is cool. It's also good for animals at the zoo!
But for the tank, it is probably not going to be too practical. You will have to be pretty far back from your subject in order to focus, and that means you will have more issues with distortion from the glass/acrylic of the tank. A tripod would be a must-have.