Turning off the pumps can help with general motion of soft corals, but won't do much to stop hand shake or freeze moving fish. Ideally you want to get to the fastest shutters speed possible, and there are a couple of ways to do this.
- Increase the camera's ISO setting. Be warned that as ISO increases, pictures taken become more grainy, depending on the quality of the camera.
- Set the camera in aperture priority mode, and set the aperture to the widest setting (lowest number). This allows the lens to collect more light, so you trade that for a faster shutter. There is a trade-off in focal depth when using a wide aperture, but that's a story for another time.
- Pay attention to exposure compensation, or the exposure meter if shooting in full manual mode. Most cameras have a tendency to overexpose shots in an aquarium, but I find that my best tank shots are taken at -2/3 stop or -1 full stop. Dialing back the exposure can be done by increasing the shutter speed, so it also reduces the chance of motion blurring.
- Use a tripod. Hand shake is a big contributor to blurry photos. A tripod is ideal, but you can also brace the camera against a solid object, or even the side of the tank (carefully). Try it, and see how much of a difference this makes.
- Shoot perpendicular to the glass. Looking through glass or acrylic at an angle introduces significant distortion. What may look ok to the naked eye can cause seriously blur when taking a photo. The thicker the material, the worse the distortion will be.
- For point & shoot style cameras, make sure you're in macro mode if working at a close distance. Macro mode allows the camera to focus closer than it would in normal operation. If you're closer than the camera can focus, everything will come out blurry.
That's just a few off the top of my head.
Roger, I might be persuaded to discuss this at an upcoming meeting, but I don't want to step on astrogazer's toes - he's been planning an online photo workshop to help everyone out.
