With that kind of water..even if you have it down to 360's, it's way too high!!! There is no telling what can be in that water, so there's no telling what has happened in your grow medium. Mg certainly looks lacking, but again..it would be hard to tell you how you may correct it without knowing what's been going in, and 500+ppm water at the start may have thrown things out of whack and until you know what's in the water, it's virtually impossible to say what you should do to remedy it. Are elements deficient? locked out due to a toxicity of another element? lockout due to ph? it's hard to know as we don't know what those 400-500 ppm's consist of.