Damn Steveo these aren't lookin good ! It must be the silica. I don't use silica but my guess is it's antagonizing other nutes. Flush good, if they're regs I'd putum in veg until they recovered. Your yield is pretty much lost right now. I found this.... Can I use too much Silica? Yes. Too much silica can lead to a buildup of salts and cause nutrient lockout and root damage. Symptoms of silica nutrient lock include browning or burning on the tips of the leaves, reduced or stunted growth and low yield.