As a large portion of the game requires you training your Magikarp, the game is set in an aquarium. With the Support Pokémon being found at the top, you can find your Magikarp and its food at the bottom. However, you have the ability to customise the aquarium with a variety of different backdrops and items.
These will provide a variety of special bonuses to your Magikarp, allowing for you to focus on the specifics you desire for the training of your Magikarp.
You can only place up to 5 different decorations at any one time however, but the bonuses you get from them are active whether or not you have them out.