OMM, Sorry to see what's going on with the White Russian, Brother. I'd give her a flush right away, then follow with a VERY light dose of nutes and see if she perks up. I'd be betting more on a toxicity than a deficiency and she should have plenty of stored nutes to live off of plain water for a few days without any ill effects.
EDIT: Actually, after looking at the leaf again, it does look like a pretty bad Magnesium deficiency. I know you said you were using store-bought R/O water. Are you adding Cal-Mag? I'd still do a flush & follow-up with light nutes (make sure to PH adjust!), just in case the deficiency was caused by PH lockout, or something, and make sure you add a good dose of Cal-Mag to the nutes.
