Yup, Dakar pretty much hit it right on the head. About the only thing that I would add to his post is ALWAYS shoot perpendicular to the glass. The more of an angle that the lens is to the glass, the more distorted the picture will be. A camera lens will pick up the distortion much more than our eyes will even.
Depending on what I'm shooting and where it is in the tank, I actually put my lens ring right up against the glass. This ensures that I'm perpendicular to the glass and distortion will be minimized or eliminated.
Here's some of my macros that I like:
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