What Are My Fish Doing?

I have noticed over the past couple of days/evenings, that two WCMM fish (not even sure it's the same two - they all look alike to me!) do what looks like syncronised swimming.

They are in perfect time with each other, rising and falling slowly together but they are flaring at the same time.

They don't appear to be angry (unless the flaring is showing that), there is no fin nipping or any fighting, so what are they doing.

I think I have mainly males although I am sure there are at least two females in the tank