Have been doing some research and has found that my boy has a lot of the symptoms of magnesium deficiency. I have been looking at supplements and want a pure magnesium supplement not something with added 'calmer'. I've looked at the equine america supplements but found they're very expensive, I've heard that buying the mag ox supplements off ebay is just the same, is it? Also, how much would a 14hh welsh d need? Think he's roughly 350kg.