I thought higher alk allows for faster coral growth. I just got my Salifert nitrate test and it shows the same ~20.Folowing. They do not survive in my tank. My last Xenia frag was "contaminated" with a small acropora. The Xenia died after 2 weeks and the acropora grew very fast.
Your alkalinity is very high and I am not sure I would trust a strip for parameters. My guess is that some tanks have the "wrong" bacterial population or a missing bacterial species.