I don't know why they're the only ones dying, but I would try to figure it out really quickly!
You can get test strips at Petco or Petsmart but I don't recommend those cuz they aren't very accurate. I would go for the API test kit. Get that somewhere other than a chain store cuz they are way overpriced.
Sorry, can't help you much more unless you can figure out if it's your water (almost always that's the case

)
Edit: And, yeah, your tank is way overstocked. Is the 10g cycled?
Edit2: Where are you planning to put all those babies btw? That's a lot of fry and they get as big as mom and dad!
