I have a single male, 13ins in a 180. He was in a 125 for about a year. I wont claim the 125 was too small, but he looks better in the 180. He lives alone and likes it that way.
My 75g is the same or wider than a 125 (18".) It is the same as my 135's, but only 4' long. How active are the managuense? Do they swim all the time?