After the aquarium is built, you can fill it with fish that you have not yet photographed. For this to work, you should already have about thirty photos of fish. When you go into the menu to view stats on a fish, select all the different ways to alphabetize the list. After you have done this several times, choose which fish you want to populate your aquarium with. When you select a fish, a different fish that you may not have even seen yet should appear on the list, instead of the one you picked. Put that fish in the aquarium and view it. Try every different way to alphabetize the list until you get the correct one. There are some fish that this trick does not seem to work on, however it does work for a great number of them.