I think that if you can get critters to reproduce in your tank that is a sure fire way to tell that your tank is mature and stable... Creatures follow the following life processes:finding food, finding shelter and breeding. Once you get the latter it shows that the creatures in your care are happy and comfortable.
Marginella snails reproduce in my tank; here’s a baby!
I find that COOL, he must add alot of personality to your tank.$60! The main thing is that you can't put anything in the tank that your attached to. He eats fish, coral, other crabs, throws sand everywhere, he's a total monster. Even goes after my hand sometimes