Instructions
Click on the "Store" button in the lower left corner of the screen.
Select the tab on the top portion of the screen representing the category of the item you'd like to purchase. You can choose Specials, Feed, Animals, Decor, Ambiance, and Tank Mates. Each tab also has several subcategories.
Click "Buy" under the item that you'd like to purchase. It has to be one of the items that can be paid for using Facebook credits. This information is clearly listed on all of the items.