I'm so sorry too, it's so sad. Wish I had a clue what was happening and could help, but I can only offer condolences this time
Have heard about quarantine but never done it . Dont have a spare setup to do it . Never had anything like this happen before
It would be worth getting a little tank for quarantine. Just a cheap second hand ten gallon would do since you have small fish. Cheap little heater, and a cheap basic sponge filter, can use an ordinary lamp for light. Seems like a lot of extra equipment, but you never know when a heater or filter will break (my first heater died in the middle of the night in winter.. not ideal!) so having a 'spare' is always a good idea. You can run the sponge filter on your main tank when it's not in use, so it can be cycled and ready when you need it, or you could use a basic HOB filter for the quarantine and transfer some of your main filter media into it. All this so you don't have to have the quarantine tank up and running at all times, but you can set it up quickly as a hospital tank, in an emergency, or when you're planning to get new fish.
Especially since you have livebearers. Livebearers are often infected with roundworms and flatworms from being mass produced in huge fish farms and exposed to all sorts of things, and it's much easier to medicate them in a ten gal quarantine tank, than have them spread that to your main tank and having to treat the whole display tank, or have new fish bring in an illness that wipes out your main tank, as you've sadly just experienced.
Plastic storage totes can be used too, I personally don't suggest them for quarantine purposes though since you want to be examining the fish every day from every angle, looking for any signs of problems, and it's harder to see them clearly through a plastic tub.