Quote Originally Posted by Catt Mandu View Post
Although I can't rule out one of the problems Ray mentioned, to me it looks like magnesium deficiency. To see some examples, Google 'orchid leaves mineral deficiency'. One of the results will be from the St. Augustine Orchid Society. Check the photos of Mg deficiency, compare to your plants, decide for yourself.
...and I can't argue with that, either.

Mineral deficiencies are highly complex and a deficiency or excess of one can give symptoms that look like another!

Epsom Salts is an easy and safe thing to test; I've applied it as strong as 2 tablespoons per gallon. If your plants do have a mag deficiency, you should see a change in the leaf color in a matter of days.