Ooo, something I actually have a bit of a clue about!
I imagine that a more-advanced camera will allow you to take better pictures. When I take pictures (of anything really, but let's focus on fish), I take into account the lighting (direction and intensity), whether the object is moving, whether I'm moving much, how close I want to get, etc.
Maybe other species are different, but my fish (see my signature) don't stay still for very long. Maybe the current in my tank is too strong, I don't know. Anyway, if you want to catch a moving object, you're going to have to increase the shutter speed. On a digital camera, that's the number that looks like 1/15 or 1/100 or 1/250 or 1/1600, and so on. These numbers mean "It takes 1/15th of a second for the shutter to close (I think)". The longer it takes for the shutter to close, the more likely the fish will move before it closes, and you'll get a blurry picture.
I personally have trouble with lighting. My tank has a 20-watt fluorescent light, but it's rarely bright enough for me to maximize my camera's combination of high-quality controls. What I mean by this is that if you want to take a really close-up shot, and want it to look sharp and not grainy, it's best to use a low ISO (My camera goes as low as 80). The problem with that is that lower ISO makes your scene look darker...so you need to raise the lighting (or maybe use a flash, but I almost never use flash). In addition, if you want to raise the brightness, you can lower the f-stop value/aperture. This is the number that looks like "f 2.7" or "f 5.6". 2.7 will result in a lighter-looking picture than 5.6. As far as I know, the aperture is mainly meant for focusing (i.e. near vs. far), but I use it mainly to adjust the lighting internally.
So, the following things affect the lighting, which will, in a way, affect the quality of your pics:
1. Shutter speed - faster will make your picture darker, but it will make it easier to capture a moving object and make it look like it's not moving. If you want to use a faster shutter speed but are concerned with how it'll darken the scene, use a lamp and point it at your aquarium.
2. ISO (film speed) - Lower ISO means sharper images, but lower ISO also lowers the lighting, so you might have to compensate by adding an external light source.
3. f-stop/aperture - This is DESIGNED for focusing, I think, but you can use it to control lighting: A smaller number will brighten the scene and a bigger number will darken it. I think a smaller number also allows you to focus on the closest objects and leave the background blurry, whereas a bigger number will balance the focus of the whole scene.
4. Type of light source - My camera has the ability to change the coloring of the scene based on the TYPE of light source: sun, cloudy, tungsten, fluorescent, fluorescent H, and automatic white balance. I push the "Func." button on my camera and it's an option near the top left. It may be different for different cameras, though. If your aquarium uses a fluorescent light, choose that setting. Of course, I often add a tungsten lamp, plus natural light from the windows, so that's a jumble of sources. In that case I personally use the "automatic white balance" setting.
Of course, you should make every effort to stabilize yourself and your camera, because cameras pick up small movements. Prop your arms up with your elbows, use a tripod, sit if you can, and so on. Figure out how close you can get to an object while remaining in focus. Use the Macro setting to do that (it's often indicated by a little tulip-shaped button or setting; it allows you to take focused close-up pics. My camera can focus with an object TOUCHING the lens, but not all cameras will do that. Mine is a middle-of-the-road camera, a Canon PowerShot S3 IS. A camera that can fit into your pocket will probably not be able to take as-good close ups, and an SLR with a macro lens will probably be able to take better shots than I can with my camera.
So yeah, fiddle around with external lighting, ISO, shutter speed, f-stop/aperture, and your camera's macro setting. Go to
www.flickr.com and search for macro pics to see what's out there, in terms of what people are taking good macro pics of. My flickr page is in my signature and I have some macros there (although very few of fish at this point), although I'm not a master photographer in any sense yet!
Also, as Butterfly mentioned, you may have to take lots of pictures before you get one that blows you away. The other day I took about 100 pictures of one big icicle, trying to get as many great shots as I could. If you want to take fantastic shots, patience, timing, and knowing your camera's abilities and limits are among the most important things.
Good luck!
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