CRS died overnight... :(

[Kinection]

I just did a water change the day before my
2 CRS died. I bought 5 and they were doing great
for 2 days... now two are dead. I have no idea how
they died. They are co-existing with 7 Fire reds
that are doing great, 6 females berried! What
do you guys think happened, my parameters are
perfect, pH is 6.8. For molting issues, I think there
are none, my Fire reds molt perfectly fine.