hmmm... to answer your sand questions- we didn't add the sand ourselves, it just happens to be in the standard potting mix we buy (also has peat moss and perlite in it). We have all of our plants in the exact same soil mix, so i don't think that's the problem- although I really have no idea what else it could be. We have a pH tester, so maybe I will test the runoff water again and see how it is. We test the plants intermittently for pH, so maybe it's time to do that again. Thanks for your input, I hope you are right cause I have no clue what else it could be.

ps: some of our other plants have strange spotting too, but instead of darker spots, they're yellow. maybe that will help with the diagnosis...