You can buy new fish and aquarium decorations in the in-game store. To do that, tap on the stand icon in the lower left corner. A pop-up window will contain several tabs. The tab with a fish icon contains new pets, the tab with a tower icon contains algae and regular decorations, while the aquarium icon will take you to themed decorations.

Please note that the decoration tabs contain their own sorting system at the bottom of the window. Use it to quickly choose the right category. For example, you can use it to find algae or decorations for a specific aquarium.

The store can also be used to store items: you can store unwanted decorations there or use it to move fish to another aquarium.