My platies are dead

Not sure what is going on put my platies keep dying. I've lost 4 in the last week. I see no signs of illness. Water parameters are fine. I've lost no other fish and I figured that if I was going to lose a fish to water conditions it would be my Blue Rams or Harlequins.

I have no idea what is going on...

I'm just bracing for something in my other fish but they seem fine. When I researched platies I thought they were going to be a nice fish. Frankly, I have come to dislike them very much anyway. They are messy and very boring. Either hiding at the bottom or in the plants. Now maybe that's because something is wrong with them but not an interesting fish.

By the way, of my remaining platies, one has an s-curve back and looks deathly - he's the one I've been thinking would die first but he's still kicking after a long time. Another has his fins clamped most of the time, another never comes out except to eat and the other big fat one is always stealing everyone food... The two that I thought were going to die haven't yet and the others always looked healthy and it was surprising they did.

46x16x24 it's a 77g with lots of cave, wood and fake plants.. i also have 20 harlequins, 2 blue rams, 6 cories and a bushynose cat.