An xsi would get much better pictures than mine. The last time i worked with it, I went out on a sunny day and set the white balance outside. when what you see looks like what it actually looks like (colors) then go ahead and try it on the aquarium. The next thing would be your ISO. You would want to use the lowest ISO setting. I think 80 would be the lowest for a 1/16 shutter speed. I usually toy around in the 1/16 and 1/32 range for SLR cameras. keep in mind, all this has to be done in manual mode. set the camera to macro and try it out. The hardest part will be getting the white balance correct