I am currently running 2 freshwater tanks... a 55 gallon and a 20 gallon tanks.  I also have a second 20 gallon tank that is part of a sump system for the 55 gallon, and is currently home to an unidentified Loach who was terrorizing my Angels!  The fin-nipper was banished to this tank over a year ago.  He seems to like being all alone in the sump tank!!  He is a very shy fish... you NEVER see him swimming around.  He's always hiding.  And if you just happen to walk by the tank and catch him out, he quickly darts to a hiding spot!  Well, that's fine with me... he earned his spot in the sump filter tank.  After all....  he IS a bottom feeder!!  :o)