For tanks, can either visit the pet shop, or you can purchase them right from your gold fishbowl when you place it in your personal space.
For more fish you can use aquabux, which are purchasable from your tank, and then you can use those aquabux to buy fish. You can also earn aquabux daily by visiting your tank every day.
The goldfish bowl is an active item so to add fish you must place the bowl in your personal space. Then exit the furniture browser. Walk up to the bowl and press X and join the game. Press X again to bring up the menu, X again to bring up your list of fish.
;^) Try interacting with your goldfish bowl. You can add fish there, and explore the menu. Give it a whirl for a few days to decide whether you want to splash out for a bigger one, first.
Then, you can find bigger fish tanks, and other pets, in The Pet Shop under the Explore tab in your navigator's cross menu bar.
It's not a store, it's a showroom/space, where you can look everything over. The store is in the space, made to resemble the register in a real pet shop.
If you are curious about the rewards for the aquariums, I made an info thread about them with pics as well:
Sony's PlayStation Home updated mission statement...
"Like it never happened"
- Stand for something better than mediocrity and don't disguise it as adequate, when clearly it is far from that to anyone willing to be objective and unbiased. -