I have a little 20 gallon sidekick tank that I find very interesting to look at. It's a simple set-up that I toss macro algae clippings into, and I just kind of let it do its own thing.
20g long:
The tank is full of various pods and Mysis shrimp, and it is the home to only one inhabitant:
White Spotted Hermit Crab (Dardanus megistos)
He's really intelligent and surprisingly brave, and in addition to the pellets that I feed him, he keeps the macro clippings nice and trimmed.
Happy hermit 
