Just like keeping fish in real life, the fish need to be fed and have their tanks cleaned. Aquarium by DS allows the player to keep the usual types of fish that one would find in a fish tank including Clown Fish, Angelfish, Neon Tetra and Guppy.
While Fantasy Aquarium by DS allows the player to keep fish which could never be kept as pets in a tank such as Blue whales, dolphins, Seals and sharks.
There are five tanks of differing sizes to choose from, which can then be customised with different backgrounds, plants and accessories such as rocks and bridges.
There are more than 30 varieties of fish to choose from in both Aquarium by DS and Fantasy Aquarium by DS and more can be unlocked though good fish care. The player can also name their pet fish. Fish can also be swapped via WiFi with the player's friends.
If the player doesn't switch their DS on for a few days or if the environment is wrong and the fish are not cared for they die and disappear from the tank. And if the wrong combination of fish are added the consequences can be lethal; for example if the player puts a killer whale in the same tank as a penguin, the Killer whale will eat it.