2 Dead pregnant platy don’t know why

I need help I have had two platy fish die within days of each other and I don’t know why yes it’s a new tank but I’m right on top of water changes twice a week 30% it’s a ten gallon had five plattys that are pregnant when the first dies it was the smallest and I was already pretty sure she wouldn’t survive the birth and found her dead behind a rock I figured she was hiding to give birth and just didn’t make it that was 3 days ago I did due diligence and did a water change just to b safe but today I found another one dead and have no clue why she was old enough and all of these fish are from one of my existing aquarium and were bred in house any help on what I can do to make sure there is no more casualties I’m a very experienced at fish keeping and this is beyond me there is no signs of disease and both fish were acting normal and seemed totally fine eating properly and then hours later dead

 

Were they pine coning when they died? Do you have pics of before the died?

 

There was no pineconing nothing visible at all they were breathing a little heavy but I chalked it up to a bit to big of a water change and the water change was after the first one died like they all was good both of them seemed totally fine and then just dead they were eating properly swimming around the tank I don’t have my test kit here but I will in the morning and get the water quality check but it should b good even as a new tank I’m on top of water changes and dose with prime when I do water changes I was thinking maybe my prime had expired and that they got chlorine poisoning from the water changes but apparently prime doesn’t go bad plus it was fine when I set the tank up few weeks ago it’s like one of them is a cereal killer
Here are pics of the three remaining plattys
The water was a little warm but nothing drastic and again that was after the first fish died

 

Are you certain they were pregnant females? Livebearers like platys are subject to internal parasitic infections like Camilanus worms. A platy with worms can look like they are pregnant due to a swollen abdomen.

 

U know I can’t b totally sure they were pregnant I wasn’t sure about the first so I asked on here and multiple people said for sure pregnant but the second had the unmistakable gravid spot amd was close to the tail how would I tell the difference?