Alaris
Aquarium Advice FINatic
Who tested the water and with what method? Did YOU test the water with a reagent kit (NOT strips)?
PWC's reduce ammonia and nitrate. A cycled tank with proper bacteria reduces ammonia and nitrate. I don't think your tank is cycled... and you added more fish. Probably not a good idea and you should seriously consider taking them back or risk losing them.
Get a test kit. Test for ammonia and nitrites DAILY. If either are over .5ppm do a 50% water change. This may mean DAILY. Get a big bucket and a gravel vac (works as a water siphon to minimize mess) and roll up your sleeves... fishkeeping is not just watching fish swim in a tank.
Amen!
The only thing that helps reduce ammonia is the little guys (beneficial bacteria) that eats it. You can't buy that in a bottle. And yes, it probably was ammonia that killed your fish as it is extremely TOXIC to fish (which is why you don't want it....)