Hmm, I have a couple. Sally Lightfoot crab when I first started out. Man those are some cool looking crabs so I get one. All is well until my fish start disappearing. We're not talking tiny fish either. One day I turn out all the lights in the room and just sit quietly. I watch my SLF launch itself from a rock at a fish, repeatedly until he catches him. At that point I get in there with a spearing tool, rescue the fish and promptly take the rock out till I find the crab. Never again. I always wondered how he grew so fast and why I sucked at keeping fish
So I get a couple of emerald crabs to help out with the valonia problem in my tank. They start out both the same size but over the course of a year or so, one stays about the same size, the other is easily 5x the size (seriously, his claws were freakishly huge!!). I called one The Hulk and the other Bruce Banner
Bruce passed on (just never saw him after awhile) and I eventually figured out that all The Hulk was interested in was my coralline algae, which I had fairly little of in the tank (as he was eating it all). He went to live in Victoria where there was scads of coralline to be had
Pom poms are my favorite, easy to care for, they survive the most horrific conditions (auto-topoff disaster, heat disaster, high salinity disaster, yes i've been through a lot of disasters
) never had a problem with them, they just keep on cheering for the home team!