First connect the game to your Facebook account. You can do that in the game settings.
Once you’re connected to Facebook, do this:
1. Tap the Friends button above the Settings button in the left part of the screen to see a list of your friends who have Fishdom aquariums.
2. Tap the Invite Friends button at the bottom to add your Facebook friends.
3. Select the friends you’d like to invite to the game by tapping [+] in the right part of the window.
4. Tap Send.