I agree 100% that you need a fixed focal length dedicated macro with some reasonably fast glass (like f/2.8). However, if you want to play around using your stock lens, you can use a "Closeup Set" to decrease your minimum focus distance. A set like this:
Amazon.com: Neewer 52Mm 4Pc Close-Up Kit Macro Lenses For Nikon D40 D40X D60 & All Other 52Mm Lenses: Camera & Photo will allow you to use your stock lens MUCH closer. before I got my dedicated macro lens, I would use my stock 18-55mm and 55-200mm with these to get super close up shots.
You will have to stack them for longer focal lengths and you won't get amazing focus (those extra lenses reduce FOV and cause slight distortion), but they are a very cheap way to start learning macro and the basics of your new camera.
IMO the biggest challenge shooting a fish tank is getting the correct White Balance for our super high color temperate lights and LEDs. A good "white card" set is in order too so you can set a custom white balance ("Auto" only goes so far - probably around 14-15k on my older Nikon).
For shooting in your fish tank, don't waste your time on the cheap Micro-Nikkor 40mm - the focal length just isn't long enough and the 1:1 ratio happens at something like 1.6" whereas a 105mm will give you like 12" for a 1:1 ratio.
If you are interested in doing macro outside of your fish tank, definitely get an off-camera light. It is the best investment you will make for your camera after you have your basic lenses. A good tripod, shoot-through umbrella, umbrella stand, and remote flash kit (2x hot-shoe to 3.5mm cable and a cable) and a manual-capable flash (cheap flash like YN-560ii) is what you will need and will run right around $150 for all of it. Check out
http://strobist.blogspot.com/ - that is by FAR the best site to learn proper lighting from. Definitely learn macro shooting and proper manual lighting before springing on a ring-mounted lighting rig or some serious CLS capable speedlights.