I read the prior thread about Cardinal Tetras dying. I am trying to establish a large school in my planted tank. What is the maximum recommended KH for them to thrive. My tap water has a fairly wide range of KH 3-6. Currently it is coming out of the tap 6 KH, I am concerned that may be too high.

I have also had trouble keeping them alive in quarantine, but that I believe was because my heater gave out and the temperature went to 69F. The remaining survivors contracted Ich. Previous posts seemed to reference that temp was critical. I throw hornwort in my quarantine tank as well, it seems to have the effect of increasing the PH to nearly 8 from about 7.5 because of all the CO2 is drained out, based on prior threads I believe that is not a factor. All other parameters are fine Ammonia, 0, NO2, 0, NO3, 10-15.

Any advice greatly appreciated

I've never had an issue relating to KH and cardinals.
As long as you are good about acclimation and drip them, then they should be fine.
Several folks I know have the largest ones I've ever seen and they are 3-4 years old now and at a KH of 9.

The large groups in one client's are at 5.

Regards,
Tom Barr

I have kept a group of 30 at Kh 5 for a year and they have done very well.

-Russ

Alan's KH is 8-9 and he's got the largest cardinals I've ever seen and the oldest.
The key was slow acclimatization.

Regards,
Tom Barr

How slow is slow? An hour, a day, a week...? My guess is that no more than a couple of hours is very good.