I do think it is environmental and that high pH may be the cause. It seems to fit from what you say. Our city water is about neutral. The additional of magnesium certainly won't hurt. I have seen recommendations all the way from 2 tbs./gal. to 1 tsp./gal. Since I use the same solution on all my tropical plants (200 orchids plus many others), I am cautious and use 1 tsp./gal. Good luck.
