Happy Aquarium: New Mystery Treasure Chests!
These chests are being sold at the in-game aquarium store for 100 Coins each. You can find it in the "Special" section, which has the tab of a yellow star. After purchase, the chest will sink into whichever tank you've got open, and stay there for a 24-hr countdown before they release a mystery item.
Just like the Happy Pets Mystery Boxes, these Mystery Chests allow your friends to "Add Luck" to them, which is suppose to help you get a 'better' item. To do this, you'll need to click on your Mystery Chest to make a post notifying your friends on your Facebook Wall.
As you can see, my Cubicus Boxfish is eagerly awaiting for my box to open. I bet he expects lots of goodies in store for his home, or maybe even new fishmates to hang out with. Who knows? If these Mystery Chests are anything like the Mystery Boxes at Happy Pets, it'll be possible to get aquarium critters too!