To elaborate:
Fish may or may not be included in your states animal cruelty laws, but usually the laws are worded to include any thing that is a non-human living creature owned and cared for by a particular person.
Aside from that, each and every state views pets, especially aquariums, as property.
Do you have receipts? Bank account activity going back a few months? Your bank can help you out with that, perhaps for a nominal fee. Also, if she admits to it, then there is probably a related breaking and entering, criminal trespassing, vandalization and burglary charges you can apply.
Mainly, you'll probably be able to get the most satisfaction out of a monetary settlement, in which case a small claims fileing might do the trick.
There are some things you just don't mess with.
Someone's ride.
Someone's aquarium.
Someone's significant other.
Someone's dinner.
