horrible miscarriage?

Philly, if you have anything that can serve as a QT I would get that "spotted" platy out of your main tank NOW. Just in case it is something contagious or communicable.

I suspect though that you are on the right track with the observation about the eaters-of-gravel. These might have eaten something small enough not to kill them immediately but big enough to cause a blockage further down the intestinal tract.

Of course my medical qualifications for diagnosing peculiar diseases come from obsessive watching of "House."
 
sounds like possible anchor worm to me
 
callamanus worms...

If the sore is on the anal hole then I agree. Best treatment is the use of a dewormer. Jungle parasite clear works well, but a lot of folks will use Praziquel which is a dog dewormer. You'll probably need to treat the tank since worms typically form cysts that hang out for a while in the water and can be re-ingested.
 
The sore is on the abdomen. The front and to the side, pretty much on the "corner" of the belly bulge. The fish is still alive and behaving mostly normally. She just doesn't shoal with the others quite as much. I don't understand the water if perfectly fine!
 
I believe only my female Platy got this and only once they were already round, possibly preggo. I'm thinking for some of them, it was the fry in them that started the problem.

Do you have any algae eaters that might be sucking on them at night?

Wouldn't you see the tiny red worms hanging out eventually if it was C. worms?
 
Is there any way to get pictures of the one that's alive? That would help a lot
 
Sadly the other one is no longer alive. I was talking with a guy at a fish warehouse-type store and he would bet that it was some sort of bacterial infection. I am inclined to agree. Either way everyone is happy now.
 
Callamanus worms look for red threads coming out of your fishes anus. Or possibly dropsy I've seen fish 'explode' from that before from the way it was described I would suspect dropsy and watch your other fish very closely because if it is it may only be a matter of time before more deaths occur.
 
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