Oops, sorry I forgot about this post, must have been a bad day at work that day.
http://www.dpreview.com/reviews/specs/Canon/canon_s200.asp
Those are the specs on your camera. One thing I notice is that your cam does not allow for a manual focus. Your cams auto focus type is...Auto focus type Ai AF TTL 3-point or center point , see which method your cam is set to use and switch to the other, most likely it is set to the 3 point by default and you want to switch it to center point. and change the metering to center weighted Then try to keep the fish in the center of the pic. Also, make sure your using your macro mode and do not try to get closer than 10cm to the fish, that is the smallest range it will focus at.
Use a tripod and the self timer, when possible, in aquarium photography your going to end up with slower shutter speeds which is going to cause blurry pics by either subject movement or camera shake. You can speed your shutter some by using your exposure compensation for the cam...This will over expose the pic a bit, but since the shutter speed was going to under expose it...it compensates
Try using the flash, make sure the cam is at an angle to the tank, you may not like the results, but then again you might, try it and see. Take LOTS of pics. For every 1 I am willing to post, I tank 2-3 that I don't like or get ppoor results, before I really got aquainted with my cam I was taking 50 pics to post 2, nothing wrong with that, pixels are free and the results will get better with time.
