Consider a soft, screw-in rubber lens shade as a handy tool. You can press it right up on the aquarium glass, to avoid a lot of stray light reflections off of the glass, which is often a big issue. The polarizing filter is less likely to be of a lot of use. if you have no screw-in rubber lens shades (they are admittedly, very "1980's-style" tools), take a dark hand towel and use it in the same way, to surround the sides of the lens, to keep stray light on the outer glass from messing up your shots.
Many newer aquariums seem to be better for photography than old-school ones. Do not be afraid to jack the ISO up!!!! it can be very dark.
Also, consider slower-speed panning, like 1/6 to 1/20 second, on some of the fish or mammals...might get some good shots!