I have heard about a custom of not going to a zoo during pregnancy, does this apply to Aquariums as well?
Does it apply to everything, for example just seeing them, or only getting real close to them?
Although there is no clear source that states that a woman should not go to a zoo during pregnancy, many women have the custom to avoid going to the zoo when pregnant. The basis for this is that whatever a woman sees during pregnancy has an effect on the future child, and therefore some women prefer not to see an impure animal (i.e. an animal that is not kosher to eat) during pregnancy. Although at times seeing an impure animal is unavoidable, e.g. in Israel cats roam freely in the streets and it is obviously impractical to try not to see them, there is still merit to avoid going explicitly to a zoo and consciously staring at the animals.
This does not apply to an aquarium. Although an aquarium contains impure fish, the custom, as heard from Harav Yisrael Gans shlita, is not to avoid going to an aquarium. Apparently there is a difference between animals and fish.
All the best