If your feeding with softened water.yes you will have problems because I do believe those units add salt which isn't good. Also rust spots on leaves does sound like calcium. But you have to be careful with a cal/mag supplement because often times it's just one and not the other so we don't need to supplement with both. But yes I would water with a cal/mag supplement as it seems your displaying a bit of both. I would water with full strength cal/mag and 1/2 strength nutes.