For a first DSLR, your best choice is to spend a part of your budget on the camera itself, and another part of it on a good lens.
Almost every DSLR out on the market is great, as long as you have a good lens to go with it. Obvisouly, which brand you merry into will determine the quality of lenses available to you. You can't go wrong with Nikon or Canon.
The 18-55mm lens that is included with most kits is just junk. You could purchase just the body of the SLR (without any lens included), and you can save yourself some $100-200 which can be put towards purchasing a good lens.
I don't know what your primary interest in buying this camera is, whether fish photography or a general all-around camera. This will be important in deciding which lens(es) are best for you.
If you are on a tight budget, and you go with Canon, you can go ahead and for a measly $100 purchase a good 50mm lens. It won't give you macro capabilities, nor any flexibility as a zoom lens, but it's as sharp as some of their thousand-dollar lenses, and is great for portrait shots (of people).
Depending on what size the fish you are photographing are, you may or may not want to look into a dedicated macro lens.
They are usually more expensive, but give extremely sharp photos and can focus up really close to the subject (useful for small subjects). They range from $300 and up.
Obviously there are several choices in macro lenses as well, but that choice would depend on how much room you have between yourself and the subject (i.e. maybe you have a very deep tank where you want to photograph reclusive fish, or a small 20 gallon with active, in-your-face fish).
I use a Nikon D70s along with a 60mm macro lens. I bought the camera with the 18-70mm kit lens (which is actually considered quite good), and photographed fish using that for a while, but after switching over to a macro lens, WOW! The subject is so much larger, and the tiniest details are very sharp.
Realistically, you WANT to have a good lens, because if you stick with the 18-55mm lens, it's doubtful that you'll see much difference in your photos from the point-and-shoot Sony. And unless you're using the manual controls on the DSLR, you've just got a big, bulky P&S.
I've just briefly skimmed over most things here, and I understand this may be a lot of confusing stuff. I encourage you to seek out other sites that are much more into photography as well as the photography of fish. They will help you out greatly.
Of course, if you wanted to significantly improve your fish photographs you could look into a remote flash unit, but that's whole different story...
