My problem is generally having to crank up the ISO enough to deal with the low light levels and the moving subjects. I also hate the lighting they use in our local aquarium as most everything is very blue no matter where I set the color temperature. Most of this can be taken out in post, but when they use blue lights it's impossible to remove it all from my experience. While a polarizing filter may help with reflections in some situations, it would indeed make the low light situation even more difficult to deal with so I haven't even tried that option as yet, but don't expect much help there either. As others have stated, try to get right up on the glass if possible and I've found that shooting at a bit of an angle can help too, especially when I can't get right up on the glass. Focus can also be a difficult issue to deal with due to shooting through the glass, etc. I've used both manual and auto focus and either way have real problems getting a sharp image. This may also be somewhat due to the low light levels. Anyway, interesting photographic challenge here and one I get a bit better at each time I go to the aquarium. Good luck and good shooting to all.