As with all aspects of gameplay in Fishville you are limited in what you can purchase at the store to spice up the look of your tank. There are three different areas at the store for you to check for things to add.
Click the "Decorations" button and you see cool items to put in your tank like a mini beach ball, a skull, and even a hobbit hole.
Visit the "Plants" section of the store to make your tank feel like it has some natural environment- your fish will enjoy it and it adds a lot of color. You can also put big rocks in your tank.
Go to "Environments" to change the background of your tank, and to change the kind of rocks, or sand for the tank floor.
When you first start playing there will only be a few things unlocked for you to buy, but earning that next item is part of what makes playing Fishville so fun!