How do I moniter a new fish if I never see it?

[new but handy]

I wanted a male/female pair but OA only had males. FWIW OA told me not to get 2 males even in my 180. They have seen males fight to the death in 300's
OA has tanks that are 4'x6'x16" and the males stay way away from each other. 1 of the males had huge bite marks on its back from 1 of the other ones picking on it. mike where did you find the mandarins? How much?