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