haven't really shot fish tanks yet, but to my photography knowledge, you should look for a lens that will give you at least 50mm. this is to prevent any barrel distortion so that your image of your fish tank will look like it has straight edges.
the 17-40mm would give you an actual range of 27.2mm - 64mm on your 450D after factoring in the focal range multiplier aka the sensor crop factor at 1.6x.









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