150 gal should be enough for a harem of Watanabe. If you want to add a female to an existing male, it most likely will be fine. Male do chase female but not to the point of injury. I had my pair since 2012. They were in a 120 gal cube. I moved them to my 320 DT and they did great there, spawn very often. About 6 months ago I found the female on the floor. I bought another female, much smaller than the male. Both doing well and spawned often now. The male is about 6 inches counting the tail runner, and the female is just a tad smaller. I think they have reach full adult size in my tank. This is not very big. Perhaps in the wild, then can get larger. The male have not grow in the last year.