My top tip is to use the Live View function if you have it.
You can determine very accurately (especially on modern DSLR LCD screens) how the shot will look. Also use the zoom buttom (x5 or x 10) to get the focusing spot-on (focus manually at a point in the foreground, typically.)
This is my typical workflow for shooting a full-tank shot.
1. Turn off filter to avoid moving plants that may blur.
2. Set up camera on a tripod.
3. Ensure the camera is perfectly square and central to the aquarium.
4. Fill up the viewfinder (or LCD in Live View) with as much as the aquarium as possible.
5. Set camera to manual mode. Focus manually too.
6. Adjust aperature, shutter speed, ISO to obtain correct exposure. In Live View you'll get a good idea before you press the shutter release.
7. Use White Balance adjustment in Live View to accurately determine your white balance. This saves lots of time in Photoshop etc.
8. Take a few shots. Bracket the exposures and deliberately under and overexpose.
9. Choose the best shot once it's uploaded onto your computer.
10. Open the shot with Photoshop.
11. Crop out unecessary background clutter i.e. above and below the tank.
12. Make your post-processing adjustments. Hopefully there won't be many because you've nailed it by using Live View...
13. Resize to 800 pixels or so along the longest edge.
14. Unsharp mask at 0.2-0.5 pixels, 100% threshold.
15. Curves to create necessary contrast if necessary.
16. Upload to Photobucket or Flickr.
17. Grab link and share on UKAPS!!
Because you're using a tripod there's no reason to rely on fast shutter speeds unless you want to capture fast moving fish. Also consider using mirror lock-up and self-timer (or remote shutter release). Although mirror lock-up is uncessary with Live View, as the mirror is already locked-up. Self-timer is good because it eliminates any camera blur caused by pressing the shutter release (this can even occur on a tripod).
Play around with different focal lengths to get difference perceptions of depth. Wider focal length (lower mm) = deeper perspective.
For 'serious' shooting I will get lots of extra light above the tank (flash or loads of nice colour renditon T5). This obviously gives room for greater depth of field (smaller apertures), faster shutter speeds and lower ISO. All this adds up to better quality images.
Finally, most modern DSLRs will shoot up to ISO 1600 or above with minimal noise, especially if you're only posting 800 pixels images on the web.