It's all about the lighting. If you don't have any wireless speedlights, then throw extra lights on the tank. Here's my tips:
w/ overhead flash:
- fast shutter speed (1/125th or faster)
- small aperture (f/8 - f/16, actual depth of field will depend on focal length of lens and subject distance)
- low ISO (100-200)
w/o overhead flash and tank lights only:
- slowish shutter speed (in general, i try not to shoot below 1/30th handheld)
- big aperture (as big as your lens can go, kit lens will be f/3.5-5.6
- higher ISO (on the D90, I don't like going above 1600)
The latter settings won't give you the best results, but it will be the best you can do until you invest in a speedlight