Help, my plants are yellow-lime-green.

Hello all,
I have read through a few of the sections in this site but wasn't able to get more than an idea of what could be wrong as an a first-time grower I would really appreciate any help.
The pics are three of the plants suffering from the same problem.
Plants are clones almost 4 weeks old.
I'm growing indoors using an Eb n grow system with hydroton.
18 hours of light per day using three 600 watt HPS bulbs about 22" from the tops.
CO2 is set at 1500 ppm.
Temp is kept between 72 and 82 degrees.
Good air movement with a steady circulating fan but this is a closed system so the air from another room passes through the ducting, through the lights then outside never mixing with the air in the room.
Watering for 15 minutes every three hours with 1200 ppm of nutes.
After feeding nutes come back at the same level of ppm but come back with a PH of 6.1 so I have to adjust the PH down to 5.8 before every feeding.
I have a total of 12 plants all with the same symptoms, growing very slowly, light green color, some leaves curl downward.
Here are the pics and any help to get these guys back to health would really be appreciated. Keep in mind these pics make the plants look darker than they are.
Thank you!!

The light in the pics is not correct, so I cant tell really what is going on.

From your description, it looks like you are over loving them.

If it was me, I would raise the light to 3 feet, stop with the co2 and cut my nutes to .9 or 1.0 EC. I don't know your conversion factor.

And let your PH drift. Start at 5.5 or 5.6 and and let it come up to 6.1 or 6.2. Did you rinse your hydrotone really well? If your PH is rising and the EC is not dropping there is something really wrong here.
What nutes are you using?

Is you rez cloudy or is there stuff growing in it? Is there white slippery powder in it as well??

This is a root prob in turn its stopped taking up nutes and if it goes on much longer they will die. You must get h202 at 17.5%(40:1) or 35%(80:1) or 50%(100:1) grade and put in the res every 4 days as it sounds like its full of nasty pathogens. When you first put it in the res it will bubble up ,killing the pathogen.

Hey Tenthirty thanks for the reply.
I changed a camera setting so this pic's a bit better on the lighting. The coloring here is very close to the actual color.
I just raised the lighting to 30" above the tallest plant (is about 38" above the lowest) but I'm gonna keep the CO2 going to help, I hope. I am adding more RO water to the mix and will lower the ppms down from 1200 to 850 or so.
The next feeding is soon so I did lower the PH down to 5.6 and I rinsed da hell out of the hydroton, spent about 6+ hours rinsing all of it half a bucket at a time.
Nutes, using
Cal/Mag, Karma, Orca, Pro gro, pure blend and thrive. I type too slow to break down the exact amounts but I'm staying within the recommended limits for plants of this age in the Veg stage.
The rez, I just looked and it does appear (brownish) cloudy, I can't see to the bottom of the barrel (it's a 25 gallon batch) but no white powder.
Any ideas are appreciated,
Thank you.

Hey Buds thank you. I'm going to put some in the rez right now.

I added about a pint (was all I had) of H2o2 to the (20 gal) rez plus am continuing to add RO water till I get down to 850 - 900 ppms. Also turned off the CO2 and opened the filtered air back into the room.
Now just a wait and see.
Thank you guys for the help, sitting here for the last couple weeks trying to figure this out has been driving me crazy. And while I hate jumping on any forum the first time to ask for help isn't something I like to do I was about ready to turn off the lights on all this so thank you. While this is the first attempt of mine to grow indoors I don't have much insight but if there's anything I can do to help you the same let me know.
Thank you!

Thanks for the input you guys. I ended up flushing the plants with just water three times and they got their color back almost immediately. I then reduced the nutes down to 400 then put em back on CO2 and now we're good.
Thanks again.

400 ppm nuts?!? Or plus water ppm?!

I still see a touch of mag def. Could be residual. better to feed light than to try to stuff the little babies.

Exactly, yea I flushed em with RO water (PPM at 26) then rebuilt the nutes back up to 400. They started showing signs of canoing (canooing er whatever, the leaf edges were curling up but no sign of burn on the tips) from heat-stress (high temps) so I turned off one light until I can have the HVAC guy install the air conditioner. The slight yellowing is just residual, compared to how they were doing they're kickin butt and are looking much better.