There isn't a significant difference between the canon xt,xti, and xsi all are very similar and each succession just adds new features and megapixel increases that are not overly critical. So if you can find an xt at good price go for it but keep in mind that the price you are seeing is just for the body or for the body and a single lens that may or may not be good for photographing your tank. Digital SLR's are great cameras especially canon but cost is very high compared to the point and shoots that are available granted there are benifits to the SLR's but most people do not need that much camera. A good P&S camera is a the Canon Powershot S5 IS about $300 and if you use it as a comparison it has a 12x zoom and an 8mp sensor and two macro modes for taking pics of your tank. Now to get the same versatility in an SLR you are looking at $4-500 for the body, $300 for a macro lens, $150-$300 for a wide angle zoom, and $500 for zoom lens that doesn't even come close to the zoom on the P&S. The prices I used on the lenses are very cheap you can go much much higher my current camera setup consists of a Canon 30D $1000, 55-85mm lens $150, canon 70-200mm 2.8 $1300, canon 100-400mm IS $1600, and I still don't have a macro lens for my tank so I am stuck with setting up a tripod 6ft away from my tank to take pics.
So take a look at any of the canon P&S cameras the Powershot S3 and S5 are the only ones with 12x zoom but you can't go wrong with any of them.
