I had to split them up

They kept fighting. Lady continually kept Perry in the corner, if he ventured out to greet a human at the glass, or grab a bite of food, or surface for air, she zoomed over there and threatened him. My husband and I imagined what the fish might be thinking of the situation. I looked in the tank at him looking back at me and said

"What's the matter, Perry, are you okay? Is your new girlfriend being mean?"
Husband: "That's not my girlfriend."
Me: "What the hell is this? It was fine living next door, but I don't want to share her bedroom."
(No, I was never intending to breed them, if you're wondering.)

Once again I saw him spar with her, buffeting side by side, darting bites at each other. They both came away with more wounds. Lady now has injuries on her mouth, and two marks on her side.

Perry doesn't look as bad, but certainly is cowed.

Instead of putting her back in QT, I made a wall. Tank is now divided.

This is Perry's side-

And this is Lady's. Yes, I favored him, because she's the bully.

At first Perry would flee, tail low and pinched, when Lady zoomed at him from other side of the wall (they can see each other dimly through the mesh)

but once he realized she couldn't actually reach him, he seemed to relax

She's wearing much duller colors now, and no longer flaring her fins or making short, quick darting moves all over the tank.

He still often seems submissive when she tries to bite him through the barrier

and darts away when she threatens too much

She leaped over the first barrier I made, out of plastic canvas scraps stitched together. It wasn't stiff enough. I think she went over the top. I netted her back into her own side, went to the store, bought a new piece of plastic canvas, came home, caught her again and put her in a box of water while I removed suction cups from the first wall and fastened them to the new one. Also softened the suction cups first in hot water, because a few of them weren't holding well. New wall seems a lot stronger. Later I was really surprised to find Perry on her side. I guess he started to feel bolder since she couldn't bite him anymore, and wanting to defend his own space, got over the top? There was a quarter inch gap to the lid. I slid the barrier up all the way, and blocked the gap now across the bottom, with stones. It seems to hold. Haven't found anyone on the wrong side since.

(PS: the suction cups are actually clear/white. Not too noticeable when the tank is window-lit, but when I darken the background to get decent photos, they stand out. I masked over them roughly with photoshop just to avoid visual distraction in my pictures).