perhaps "Mulder's Chart" will help? it's here:
http://www.nutri-tech.com.au/downloads/Hydroponic%20Program.pdf
(sorry that should read "...downloads/Hydroponic%20Program.pdf")
it shows how nutrient imbalance may effect the uptake of minerals.
in your case oversupply of Potash, Calcium, Zinc may cause Magnesium lockout.
Nitrogen and Phosphate assist Magnesium.
Zinc gives Magnesium a double whammy because as well as militating against Magnesium it militates against Magnesium's 'friend' Phosphate.
so, just wundrin whether your water is coming thru old plumbing with galvanised pipes. excess Zinc may be entering your system this way. and you wouldn't be mixing nutrients in a galvanised container?
cheers!