4th week flowering yellow leaves

[BlackShadow]

After leaving for a two day vacation from the grow site I returned to find my hydro system nearly dry and all seven of my plants quite thirsty. I presume they only got half of that day's normal feeding so it wasn't a crisis or anything. Adding fresh water and new nutes (Fox Farm) four of the seven went back to normal, however one started to develop a great deal of yellowing at the top leaves, bottoms are still a helthy green, and two others went for a full body yellowing about 40%. The system is a PVC top feed system with Fox Farm nutes, followed to the schedual for the most part and in the 4th week of flowering. The lamp is a 600 watt HPS with cooling reflector and fans that exaust the hot air out and an AC unit keeps the room at a temp normally between 86-90 F with occurances of 100 F at max only lasting about an hour or so. I'm not sure what the problem is. The water is reverse osmosis filtered stable ph and hasn't changed throughout the 9 weeks of life. Pics included were taken under the HPS light which I know is a no no, but thanks to paint shop pro i rescaled the color balance so what you see in the photos is what it looks like under normal lighting.

[SmknVTEC]

For the tops of the damaged plants I would top those off. You have major heat stress and light burn. Topping will give them some distance from the lights and bud coalas will really grow in. Do you know the pH? Make sure that's in range for hydro 5.2-5.8.

[BlackShadow]

thanks, the Ph is 6.4, I solved the problem though. Turned out to be major salt buildup causing nute block...a day of ClearX running through the system did the trick, had salt crystals the size of coarse sand, but I'm gonna take your advice as well just to be safe.