To add to what Brie and I have said about lenses....
Think about your budget. To get the crisp, detailed Macro shots, will require a Macro lens. Yes, you can get decent and really good shots, without a macro lens. However, you can get much more detailed and much better shots, with a macro lens. Keep in mind that a good macro lens will cost you as much as the camera body...lol.
When it comes to a DSLR, you're spending big money. The lens selection is one of those reasons. When it comes to lens selection, one of the most important things to consider is how "fast" that lens is. The lower the aperture number (Fstop) the larger the aperture can open, the faster the lens is. "Kit" lenses usually aren't all that "fast" with most of them having the lowest aperture of 3.8 or 4. This limits the speed of the lens, which has to be compensated for, by either slowing the shutter speed or raising the ISO.
The higher end lenses are "faster," with the lowest apertures being 1.8-2.8. These lenses typically cost more, but allow you to take your photos with a lower ISO and faster shutter speed.
Now, with P&S cameras, the newer models have excellent White Balance controls, but still suffer with aperture and shutter lag. Shutter lag will really tick you off, when you're trying to take pictures of fish. Shutter lag is the time between you pressing the shutter button and the camera actually opening the shutter. With a dSLR, it's instant. With P&S, depending on model, it can be anywhere from a fraction of a second, to a complete second. In that second, the fish is halfway across the tank...lol.
Shutter lag is where P&S cameras are weak. Close up shots is another weakness they have. P&S cameras have 2 different types of Zoom. They have an actual Optical Zoom and then they have a Digital Zoom. When using one, try to stay out of the Digital Zoom range. Digital Zoom isn't a true zoom. It's a software zoom, similar to cropping. You'll lose detail quickly. So, when looking at P&S cameras, look at the Zoom ratings. You'll see cameras that say a 16X zoom, BUT, you end up with an actual Optical zoom of 4 or 6, with the remainder being a digital zoom. You want the highest actual Optical zoom you can get, to allow you to avoid the digital zoom.
The Canon G series of P&S cameras are excellent and have very low shutter lag. Blazer has some Macro shots, taken with a Canon G10, I believe, that blow anything I've seen, taken with a DSLR, away!! (Except some of Blazers shots, taken with a DSLR...lol)
Another thing. Don't get too hung up on Megapixels. With P&S cameras, sometimes they try to pack too many megapixels onto the tiny sensors. This will really cause a drop in quality. Sometimes this is true with DSLR cameras, as well. Unless you plan to print pictures, poster size, you really don't need anything over 8mp.
For years, I shot with the Canon Rebel Xt and got great results, even shooting Macros. The Rebel line is now at least 4 generations past the Xt, with the T2i. A few months ago, I upgraded to a Canon 7D. It has all kinds of bells and whistles that the Rebel line doesn't have. However, the Rebel line is capable of taking just as good of pictures.
I'm assuming a tripod is highly recommended?
