Make sure your lense is right up at the glass to prevent picking up reflections. Sometimes a slight angles is needed.
Is yours a model with a macro function? If so make sure it is engaged for those close ups as it makes a big difference in the clarity of close up objects.
why not snap a couple for reference and let us see how you're doing ? :wink:
Here's some suggestions (I'm a Nikon shooter too):
Use higher ISO if necessary and don't use flash. Flash photography is never ideal for aquarium photography due to flash bounce and correct color rendition. When shooting without flash, the lens element should be as parallel to the glass as possible to get the best shots.
Set the manual WB to get the closest possible colors to real life. This will take trial and error.
Turn off the room lights so the only light source is your tank lights. This minimizes glare and improves contrast.
Don't try to shoot too far into the tank. Water, while it might look clear to our eyes, starts to haze up photos after a few inches. Sometimes, the AF will have a hard time tracking subjects too deep in the tank.
Use tripods/monopods for a steady shot. You will probably have to shoot with a fully open aperture to get reasonable shutter speeds.
Here is a trick you might want to try with your Nikon - using an SB-600 or 800 flash with the SC-29 sync cord. The IR focusing of the flash is moved to the hotshoe portion of the SC-29 so it will still work. I bought a little plastic handle with a tripod mount screw on it, and attach that to the end of the Sync cord that attached to the flash. I can then hold the flash off to one side, at any angle, even above the tank. I then just fire away, moving the flash around until I get the exposure / shadow quality that I'm looking for.