Ah, there's your problem. A couple actually.
The tank is over-stocked if you have platies in a ten gallon, and 70 degrees is WAAY too cold for them. 76 is about right for platies, they like it a tad cooler than other livebearers who hate it below 78. Platies will enjoy a temp six degrees more than what you have them at. At 70 degrees you've slowed their metabolism and opened them up to all manner of illnesses, infections and parasites.
Platies need to be kept in 15 gallon tanks as a bare minimum(bare minimum meaning live-able, but not preferable to the fish. Only three or four should be put in a tank that small...and they can get on with a group of three but would rather have a group of four or five). It's preferable to have them in a twenty gallon tank in a group of 4-5, they just do not do well in ten gallon tanks at all.
What happened here was you likely had a build up in waste(that's your ammonia at the start, then it goes to nitrites and nitrates, ammonia should be zero when you have fish always, nitrites too, nitrates should not go above 20ppm and are also best at zero), because you had platies in a ten gallon tank..which they can handle for a short period, especially as youngsters...but not as adults or older juveniles. After a few months of them doing "Fine" they suddenly start dropping like flies and getting sick, even with exceptionally great care to your tank.
Unfortunately I don't think any of your platies will survive long here. You may be able to save the ones you have left, do large water changes, get a bubbler, and use Seachem Prime to treat your water. It stinks! It does, but it's really awesome stuff that may save your fish until you get a bigger tank or find them new homes.
Now once that tank is settled, either cycle a new larger tank, or find new homes for your platies. The ten gallon shouldn't have those in there, it's just too small and can't handle their bioload and group needs. If you want fish in a ten gallon, I would suggest either a single male or female betta, or four guppies(this is touchy...they should really be in fifteen gallons as well), or Endlers Livebearers(not the guppy crosses, N-Class Endlers like black-bar Endlers are awesome and MUCH smaller than guppies even). I would honestly suggest male endlers, males rarely even reach the inch mark, usually staying at 1/2-1/3 inch though some can hit the inch mark, females are much bigger...if you want to breed them you can. And unlike with most livebearers, you can have more males than females here, they aren't as aggressive as male guppies...but it happens so be mindful. Otherwise I'd just get 5-6 males. They ARE pretty, lovely little fish, and a small school of them will make the tank seem active and not empty.