How big is this tank? 5 ppm
Nitrate, unless you do a lot of water changes, that tank may not be fully cycled. You nearly doubled the population of the tank in one day, that's a huge ammonia boost.
I never trust "perfect" water quality numbers, they say nothing about the ammonia processing capability of your tank. I prefer to track ammonia and nitrite over time, and see them spike and come back down. Given your current death rate, you will likely see ammonia spike even if there was no previous problem, so that is your first problem.
Do a bunch of large water changes, then let everything stabilize for a few weeks. Test ammonia/nitrite every few days to make sure it's stable. Next time add fewer fish at a time, just 2-3 per week. If you have to add a lot of fish at once because you're mail ordering or taking over someone else's fish, prepare the tank ahead of time by overfeeding before the new fish come, and underfeeding when the new fish come, to try to slowly ramp up the ammonia load instead of all at once.