You have your own aquarium. Everyone starts with a the same "small" instance.
The aquarium must be kept clean and can be customized with fish and interior. In the 'in-app.' Store you can buy more, better, crazier and fish and fish food.
The aquarium must be able to send push messages if it becomes - for example - too dirty.
It must be possible to clean the glass from the outside using a sponge.
Rare kinds of fish as well as the best food is only available in the store.
The system shell be kept very simple. And you must be responsible for Artwork also.