One tip, try a much faster shutter speed (set the camera in shutter priority) when taking pics of fish, or other things that tend to move while taking the picture. That way, the whole fish will be in focus.
wow I am sooooooooo jelous of you right now I had the chance to use a 10D about a month ago and I have wanted once since but my wife will not let me spend the cash on one. Oh well I guess I just have to look at great pictures from them untill I can find someone that will sell me one for pocket change that my wife will not notice missing