Aquarium photos are difficult if you are not able to adjust all the settings in your camera. Any there are soooooo many reasons why your pictures can be blurry.
First, the view screens on most cameras, even the good ones, can give a poor preview of the actual image when its captured. So, the first thing that may be off is the minimal focal distance. You may be too close and you may not have noticed in your preview.
Second, you may have motion blur. BlakeJohn mentioned that as far as using a tripod. That covers the motion of the camera.
One of the problems with taking photos of aquariums is that aquariums are lit mostly in the blue spectrum. Camera sensors don't really take color pictures. They measure the luminance levels of the incoming light. That light is filtered through colored filters. The photo cells are arranged in a grid which has 2 green for every 1 red and 1 blue. So when taking pictures of something that is lit by a blue light, the camera sees it dimmer than something lit with <10,000 kelvin because of the lower number of photocells measuring the luminance. This is going to cause the camera to raise the iso, open the aperature, and slow the shutter speed. A slow shutter speed is going to cause subject motion blur. There are ways to compensate for this, but you need good light metering, and control of your f-stop, iso and shutter speed.
And of course when you zoom in, you have less light coming into your camera. The small sensors on point and shoot cameras need lots of light.
So basically, its about motion, focus and light. Fix them one at a time and you'll learn why your not getting sharp pictures.
Mike
