Well, funny thing happened:
I moved all the fish but one female out of the 20.
The Female was hiding in a polyresin ornament in the 55 gallon tank, so to move her I had to move the ornament. I put it overnight in the 20 with Cthulhu (the male). In the morning, I didn't see any one in the ornament, so I put it back in the 55. Later that day, I see the female in the 20 - good!
Later that evening, I see Cthulhu hanging out in the 55.
I've tried to get him out, but there is no way other than installing the ornament in the old tank permanently, which I have no desire to do. We'll see if romance blossoms in the 55.