I can't think of anything offhand. If you are running the GH 3-part, look on the back of the 'micro' bottle ( I don't have my trusty GH Micro at hand at the moment or I'd go check for ya) and if it has a significant calcium content, you can mix up a spray bottle at about half the strength it gives for the res, and do a light foliar feed to halt the spread of the deficiency until you can remedy the source of the problem.