Use the technique described in the chemistry thread in my siggy to flush down to a corrrect pH of 6.7.
ONLY use RO water if you are prepared also to ADD CALMAG to 250 ppm before adding nutes.
That looks like overfert/soil salting, and you will need to flush and go to a correct bloom fert to finish.
Epsom salts should be used with caution. A general metlas lockout like that- grab some spray-n-grow next time you are at the hydro shop. The damage won't heal, but a correct flush and feed, and a light foliar spray, will stop it spreading.