So I managed to get the LFS to order me in some golden pencil fish after searching around for them for a couple months since starting my tank back up. Well, the research I did on the fish seemed to show that they weren't any harder to take care of than cardinal tetras, of which I have a school of a dozen. I picked up 8 to start with since they're pretty small and require a school to feel safe. Two died within 12hrs and then the rest seemed alright. So I took the two dead back to the store, got them swapped out, but also picked up 2 more, bringing the total to 10. Now three days later I'm sitting at 6 fish, almost one fish has died per day. Before I added the fish I had performed a water change and made sure my water parameters were in the clear, which everything checked out. All my tests show my water is still pristine, so why are they all dieing on me? I keep an acidic soft water aquarium which should be replicating the native environment for these little guys. The 6 I have left however have all colored up really well and are eating well, so I hope I don't lose anymore. Is there something wrong with my tank, or is just because the fish are from a chain store and who knows where they came from?

Water parameters:
pH- 6.0
nitrite- 0
nitrate- 0
ammonia- 0
kh- 71.6