From Terraria Wiki
Cages cannot be opened, broken, or un-crafted, so once a critter is crafted into a cage with the Terrarium, that particular Critter can no longer be released.
- Some critters such as the Butterfly and the Firefly cannot be put in Terrarium, but in jars, bottles, and in the case of Goldfish, in Water Bottles.
- On Blood Moons the Penguins and Goldfish don't go corrupt.
- To cage every critter in a terrarium requires 15 terrariums, which makes 240 pieces of Glass (or 480 sand)
- The plural of "terrarium", besides "terrariums", is "terraria", which is also the name of the game.
- 1.2.3: Introduced.