Hi, the same thing happen to me, first it was the fish, now its the photography, im also new to the dslr market and just bought myself a Canon 450d which i absolutely love, Canon have released other cameras after this one which are the 1000d and i think the 500d these being entry level for easiest transition from point and shoot to DSLR.
My best advice to you would be go and check out all the brands ie Nikon, Sony, Olympus, Canon etc and find out which one you like the feel of most, i ended up going for the Canon after spending a good hour in Jessops just checking different models.
Ask yourself if you need live view, also what lenses are available for which camera?? I have just spent another £379 on a Canon 70-200l lens, believe me it a cheap hobby, but very satisfying indeed. I find any spare time i have im taking pictures of Landscapes, street photography, portraits, ive even just shot my first wedding as a second shooter. Really enjoyed it.
I also got a standard 18-55mm lens with my camera which is okay, but i can gaurantee that you be wanting to buy more lenses. If you are wanting to take shots of youre tank you will also need a tripod, to prevent blurry shots on them long shutter speeds. All cameras come with pretty much the same dial, Av mode (aperture priority), Tv mode (shutter priority), Full auto, Manual, my 450d iso goes to 1600 but i believe there are other that go up to 3200 so basically you can shoot in darker areas.
Also get yourself over to photography on the net, really helpful forum, its a Canon forum by the way (sorry Nikon users) i have to plug my side
Hope this helps a little, how much you looking to spend by the way????