Help Me With These Unexpected New Fish?

I suggest getting a wet chem test kit, strips are rather inaccurate. To keep the platies healthy, do a 10-20% water change every other day, or every day if they're looking ill. Feed once a day, and only a little bit. Also, add some live plants, java moss is a great nitrogen vacuum. If you are determined, I bet you could keep them alive. Platies are tough little guys, but again, it'ss be very stressful for them so do whatever you can to make them more comfortable. But don't fall for their begging, only feed a tiny bit once a day to keep ammonia as low as possible.