I'll like the entry level Canon dslr's. I own a Rebel XS. Nikons are nice too. The key thing is to head to a store and play around with them a bit. Some people like Nikon's feel or Canon's or vice versa. I went Canon since most of my friend's have Canon DSLR's so I can borrow their lenses and accessories.
The key with taking photos of your tank is to know how to use manual settings, use a tripod and patience. I have taken great photos with my DSLR and my point and shoot. With anything knowing what you are doing makes all the difference. DSLR's in the long run will give you more options and flexibilty but are bigger, bulker and can get quite costly every time you want to buy a new lense or accessory. Point and shoots can give great results as long as they have lots of manual settings and have macro focusing options for close ups of corals.