You'll raise a baby fish into an adult fish if you feed it everyday.
If you click on a fish on a store, the game gives you the option to give them a name and choose a gender of it.
As you soon purchase a fish, you'll need to feed it to let it grow up.
Without feeding a fish, the fish stops growing and it survives.
It might take you a lot of months to reach Level 80. I am sure it takes more years.
Some items cannot be purchase by coins, so they can only be purchased by pearls or Facebook Credits.
You'll have to invite friends to get a bigger tank and the game cannot let you buy Big Tank items. Blue Whales, Honu Turtles, Great Sharks, Spotted Dolphins, Manta Rays, Alligator Gars and Humpback Whales are the animals that the game cannot let you buy.
In the older versions, some fish require tasks to do. The first is the Banggai Cardinal. To purchase this fish, you must share your email address with Happy Aquarium. The second is the Cowfish. To purchase this fish, you must be a fan of Happy Aquarium.
Happy Aquarium Wiki