Leaves Turning yellow (pics included) please HELP

Hey RIU, i noticed the other day my leaves were turning yellow on the tips, Right now there under 20/4 400w HPS lighting and are getting 1/2 strength nutes with every other watering...Any help would be appericated. Much Thanks RickRolled

looks overfed to me. try flushing and leave it to dry out no feeds till then. if it doesnt spread/worsen then you solved the problem. worst outcome is you have a clean medium to work with. if you flush and see improvment after no feeding, give it 1/2 strenght of what you were giving before and work your way up slowly again.