You will always get a blur shot as long as you keep using an ISO speed of less than 400. If you’re going to use a digital, you need to set the ISO speed to 400 or 800. Been far away or too close to the tank won’t mean anything as long as you SET THE ISO SPEED. Buy a film camera for $10 and a film ISO 400 and it will get better results than your Digital KODAK. If you are at low light with a slow film the shutter will stay open longer and it will blur anything that moves. A fast films more sensitive to light (400, 800, 1000, etc). A film of 60, 100 or 200 won’t do the job unless the room has enough light for the sensor to set the shutter speed. You can set the shutter speed your self but you might not have the camera that will allow you to do that. Either get a Canon EOS 10D or Nikon D100 which are digital and then set the speed manually or use auto. Select the Running little man on the dial. You can’t fallow a fish and then take a picture of it. That will blur the background, which is a cool effect but it won’t display your rock setting or corals.
If you’re away from the tank use faster shutter speed or FAST FILM
If you’re very close to the glass, use as much light possible and a speed setting or film speed of 400
This is really NOT hard at all.