You pretty much have to shoot as perpendicular to the glass as possible in front of your subject. I've never really understood the popularity of curved glass aquariums for this reason - eventually the novelty wears off and you realize you can't get a undistorted photo of your inhabitants. Also consider that these tanks are more prone to failure than your standard square tanks.
What's even worse are cylindrical tanks - nothing like the fun-house mirror effect with whatever you're trying to photograph. A prime example of (IMO) "just because you can, doesn't mean you should." I remember visiting a public aquarium a couple years back that had some (otherwise neat) clownfish/anemone displays but they were completely unphotographable because they were in tight cylindrical display tanks.
Maybe there is some kind of post-processing algorithm you can apply but I would think it would be heavily dependent on the camera position and angle which isn't going to be readily available to work with (beyond guessing), but as far as technique goes, I think you are pretty much limited to "try to avoid shooting at any kind of angle."