Working fish: Ok, so we all have a wide variety of fish in our tanks, but what fish do you have that are there to perform a specific task?
First I have my Filefish. He's there to eat the Aipstasia. If I look very very hard I may see the odd baby Aips in the tank, as the Filefish does a great job of eating them and keeping them under control.
Next is the Cleaner Wrasse. I don't think I have ever dived anywhere without seeing these on the reef. To me they are an essential member of the community. Not the best looking fish but essential.
Orange Spotted Goby - He's there to keep the sand clean and does a very good job.
Harlequin Shrimp - The Hunter! He has to keep the population of the Asterina Starfish in check. Never see him during the day as he tends to hunt at night. In fact I can go weeks without seeing him.
What 'working' fish do you have?