My 'natural system' pico aquarium. Currently just home to some sponges and two hermit crabs, and the chaeto of course. And a lot of algae. Will add some hermit crabs and an anemone.
The two crabs got along just fine, though earlier I saw this and was like... 'where is the crab... '
Here is the new shell it took over. The one is the very center of the image. The other hermit crab is looking down from above. And yes, the sponge has green growth on it. D: