I am by no means an expert, but I enjoy photography. Here is what I have learned so far.
Macro lenses wont help you take sharper pictures, if anything it compounds the problem. Great macro shots are a real challenge! Without coming across as a complete jerk... I think you should spend a little more time learning your camera before investing in another tool. In my experience shooting aquariums is quite the challenge. Honestly, I think it might be easier to shoot the milky way. Maybe practice on easier non moving subjects in better light until you get the hang of it.
1. Patience. I can easily take 500 photos, and, if I am lucky 1 or 2 are "decent". Not even close to professional ...rather acceptable. Look at your results on your computer because everything looks sharp on your 3" lcd screen.
2. I have achieved my best results when I shoot in manual. Don't adjust your focus all the time rather wait until your subject enters the frame. Which leads me again to point #1. Longer working distance lenses will allow you to back away from your glass, increasing your working distance and allowing the fish to not feel threatened. I will come back to this more in detail.
3. I recommend always using a tripod. It will require more patience and you will have tons more blurry shots. If you have to hand hold the camera, bob in and out mimicking the subjects movement. Yes, you will look like a freak (as my wife) but it helps get more shots in focus.
4. You can never have too much light. Invest in a remote flash, place it above your tank, diffuse it. This will let help you use lower ISO, faster shutters, and higher apertures.
5. Your lens will have a sweet spot for sharpness. Find out what it is and work backwards. If its sharp at F8, you know that you will need to balance your shutter somewhere faster than 1/60 of a sec and your ISO as low as possible to get a proper exposure.
6. back to my point about working distances: most macro lenses shoot 1:1, the only difference between them is the working distance. Your cost will be in both the working distance and the speed of the glass. That faster the glass the exponential amount of light it will gather and the faster your shutter time can be, keeping the image properly exposed, and sharp. Here is a list of working distances.
http://www.nikonians.org/forums/dcboard.php?az=show_topic&forum=169&topic_id=4276&mesg_id=4276&page
If you want to try macro on a budget you might want to look into "extension tubes" and "reverse mounting adapter". Both tubes and reverse mounting work best with a 50mm or 35mm prime lens if you have one laying around.
Good luck!