But you're right, using flash destroys the photo! All I have is the original Digital Rebel and I was still blown away how little noise higher ISO had when compared to point and shoot cameras.
But you're using a freaking awesome lens! When I read "L", I knew it was an awesome lens! But then I searched for the exact model and I realized it's way beyond my league! There's no way around it. Every amazing fish picture I see on this or other forums, involve a high-end lens. I guessed a macro lens, cause I saw a lot of nice pics with the Tamron di Macro lens.