They are dying because your tank is not cycled the red is ammonia burn. Do a large water change immediately.
When u bring the remaining fish back and "raise cain" they will ask you for a sample of your aquarium water which with an uncycled tank will show ridiculous amounts of ammonia and they will tell you this is the reason your fish died and not refund you money. I would do and immediate 50% water change and try to find some established filter media from a LFS or friends tank. Then test with an liquid test kit until parameters are at an appropriate level before adding any fish. The fish store is running a business and obviously doesnt care about anything more than getting you to buy. As a fishkeeper you alone are responsible for educating yourself and prepping a proper environment before purchasing.Take remaining fish back to pet store, raise cain demand money back. Drain and clean tank, gravel, refill with ro water, and some good used aquarium water. Plant several plants, let pumps run a week check water every few days until it gets right, then add fish.
I've killed a lot of fish figuring this out.