Are My Fish Pining Away?

After two births each by my black molly and red platy, I now have over 20 fish in my aquarium. I decided I didn't want any more births, so two weeks after the last births, I separated the male and female fish. I kept the two females - one black molly, one red platy - in the 20 gallon tank with the babies. I moved the males - 1 black molly, two red platys - to a 5 gallon tank.

Immediately, all five fish went into withdrawal and hid for a couple of days. I guessed they missed each other, but I thought they'd adjust, especially because they were eating well. Slowly, they started swimming about again. It's been a week now and I thought they looked okay.

Yesterday, the black molly started to bury herself into the rocks and seemed to have trouble breathing or staying upright. By evening, she was dead. I waited a few more hours to see she would revive. When I nudged her, she was lifeless, her eyes closed. I removed her from the tank and flushed her to the watery grave in the deep blue sea.

This morning, my red platy had wedged herself into a plant. Her eyes were still open but she missed the morning feeding. She looked dead too as she did not move when I rubbed the side of the tank.

The black male molly is having trouble swimming in his tank.

I am puzzled by what's happening to my fish. Are they dying of loneliness for their mates? Were they old fish when I bought them and it's just time to go? The female molly had always had periods when she'd sit at the bottom of the tank and wait. The male molly has had trouble swimming but would recover after a couple of days. The red platy has always hid. I thought it was because two males were too much for her. Maybe it's the added stress of the separation.

Just now, I was about to remove the red platy from the tank too. But I couldn't find her. She had moved to another spot. So she's not dead yet. Just hidding again, maybe this time from the shock of her friend dying. And did I prematurely flush her friend away?

So I am thinking, Do fish have memories and feelings? I google the idea. Sure enough, there are articles written that claim they do, that fish can be trained to have certain behaviours. I wonder how long a memory retention they have.

I feel so bad now, because I am intuiting that fish have feelings and may even be highly sensitive. I don't want to bring the fish back together. I don't know if the red platy in hiding is sick with disease, and if she is not, I still don't want any more babies. I have turned into their cruel goddess.