Well that all depends. When I go to the zoo or the aquarium I’m very happy that the snakes are behind glass and always excited to watch the fish. When it becomes your enemy is when you are shooting the animals behind the glass. If the scene is too dark and you have to use your flash DO NOT shoot straight on to the glass. All you will end up with a big bright light spot and that’s it. It’s because the flash bounces off the glass right back into the camera. The way to solve this problem is to shoot at an angle to the glass. That way when the flash bounces it will bounce away from the camera and still light the glass cage.
An even better way to shoot through glass is without a flash. Use a tripod instead. Most aquariums have wonderful lighting in the tank so take advantage of it if you can.