You can foliar spray with 1tsp of baking soda to a quart of water. If indeed that's your problem this will give a quick fix. I've used CalMag at full strength and half strength with same results. But I'm wondering why you have 1500ppm of runoff??? Sometimes this measurement is affected by any solids you've given, like compost teas. You can filter them out with a coffee filter. Otherwise it seems to me maybe a lockout as opposed to a def. I would ph my nutes at 6.5, it's the best ph for soil.