Pocket Fish is a relaxing game for the iPhone/iPod Touch. The game makes it easy to own a fish tank, no cleaning necessary. Granted, in this fish tank you have multiple fish, but can only see one fish at a time, so it may not be a great substitute for a fish tank.
The game, initially, started with four fish, but there has been an update and there are now eight fish available. Players have access to all eight fish right away. None of the fish have to be unlocked in anyway. The fish are in sets of two with a male version of the fish and a female version of the fish.