There are several possible reasons why your girlfriend didn't invite you to the aquarium. Men and women are different in the way they think so it is best not to over analyze the situation or you can drive yourself crazy. Here are some possibilities:
- Maybe she wanted some time with friends.
- Do you like aquariums? It's possible she didn't want to "bother" you with it.
- She may have a surprise planned for you later.
There's no way to know the real reason unless you ask.