I think most have a CUC added for that purpose. As far as fish go, I would never add one to do a specific job. As has been mentioned, that can backfire. However, if a fish I like happens to benefit the tank in some way, that is a bonus.
I couldn't do it without my 150+ Nassarius snailToday let's talk about the fish and inverts that really earn their keep! They are the best bang for the buck when it comes to housing them in exchange for the "services" they provide!
What are the MUST HAVE workers (fish,inverts) in your tank? Please tell us why as well!
image via @Mike&Terry
This guy is my algae eating champ. I also have a tuxedo urchin to get into smaller spots.
I had a problem with algae on my sand bed and then I got two conchs and they are doing their job keeping the sand bed virtually spotless. Also I housed a diamond back goby but, after awhile he started to move too much sand and also started playing football with the small hermit crabs so I had to evict him. In my fishonly tank I housed a sea hare and he did a real good job taking care of algae not only on the sand bed also all my rock