There are definately some tips and tricks to getting decent and great pictures of your tank and inhabitants... Many will depend on your camera and features it supports, what do you have?

Some general tips that apply to all;
- thoroughly cleaning the glass both inside and out.
- disable the flash.
- macro mode.
- use a tripod.
- getting your camera lens as close to the tank as possible to allow it to focus on the subject you want and not the glass makes a huge difference.
- for shooting critters that are not in motion try the self timer or a remote trigger to prevent camera shakes.
- moving critters require a faster shutter speed of not less than 1/60 second or you get a big blur, even some can't be avoided so bumping up the ISO may help a the loss of super fine quality.
- if it's an auto-focus type try moving your subject slightly out of the center of the frame if it's not focusing in well.

If you have a higher end camera like DSLR, or even a 'point and shoot' type that supports full manual adjustment of things like ISO, shutter speed, & F-stop (aperature) there are a lot of ways to increase the odds of getting a crisp clear picture. The same principles from the analog SLR days still apply (yes I still own an old 100% manual SLR (but the processing fees make it cost prohibitative, electrons are free!) pretty sure I still have these straight (i'm sure Perpetual98 will jump in here to lend a hand if I goof them up, or not he's got macros down to a science). But longer exposure times and/or increased aperature results in a longer focal length (more of the foreground and background will be in focus), shorter exposure times will result in the subject being completely in focus and back/foreground to be out of focus (shorter focal length) ie..fish swimming or corals waving in the water. The lower the ISO setting the slower the shutter speed but more detailed the image, and of course the inverse holds true as well, a higher ISO means a more granular image and narrower depth of field.

Some of the best settings for a given shot can be found by setting your camera up on a tripod fixed on the same subject. Take a couple of shots and record the settings, each series make your adjustments and write them down, continue until you have a good amount of samples, pull the down to your machine and from there determine what works best for that type of shot and move on to the next type of shot. A lot of trial and error here, but you can really improve your odds in getting some really nice photos.

Also remember this, even if you have 1000's of watts of light over your tank, the ambient light in the tank percieved by the camera is going to be a very low light shot.

Hope some of this makes sense and can be of some help. I really didn't mean to write a book, but this can get rather detailed.

Why not post your camera make/model/specs and some sample pics with any known settings (even the ones you don't like) and we can help diagnose from there.