My plants are gradually yellowing. Can you help me?

This is my first grow. I am using coco coir and General Hydroponics nutrients (Gro, Micro, Bloom). So, I've made some changes recently and these pictures are from a week ago. I have two fluorescent bulb fixtures (like in offices and schools) holding two bulbs each. Each bulb emits 2700 lumens at 5000 Kelvin. Until the day after I took these pictures, they were hanging about six inches above the plants. There are two rows of pots, and I have a light fixture over each row. Now, the lamps are sitting about a foot and a half above the plants. At about a week and a half into the grow, the first adult leaves of three of the plants began to yellow between the veins, and claw, so I went from half strength nutrients to full strength. The day after I took these pictures, I ran two gallons (pH about 6) of water through the plants (there's five), and started using half strength nutrients again. I don't know how soil pH works in hydroponics, or how to adjust it. I have a soil pH meter, and when I stick it straight into the soil it reads 7, all around. I use a pH test kit to test the water my nutrients are in, and get it to about 6 before watering them. At the time I took this picture I was watering them with nutrients every day, but now I'm doing it every two to three days.

There's yellowing between the veins on the older leaves, and they claw. Strangely, the tips remain dark green. New growth has looked great this entire time, and layers just keep popping out. It's just that the older leaves are gradually yellowing. I think it's been slowed since I adjusted the lights and cut back the nutrients, but, it's still happening.

Looks more like pH issue than anything, but I don't do coco. The lights really should be changed, 6500k for veg, 2700k for flower.

Use this chart for knowing where to keep your ph. http://www.420magazine.com/gallery/data/500/Nutrient_Chart2.gif

I keep my PH strictly 5.8, but I run a ebb and flow table.

Your plant will let you know when its not happy, that is either a deficiencies (not getting enough) or a lockout (can't get any). That chart shows the lockout range, my best guess would be PH issue also.
https://www.rollitup.org/marijuana-plant-problems/537558-cannabis-nutrient-deficiency-info-printable.html

What I learned is heavy quality measuring tools, digital ph meter, digital EC meter, digital temp ect.

what brand of coir ?
https://www.rollitup.org/drain-waste-hydro/639193-coco-tips-techniques-facts-you.html

did you rinse the coco ALOT prior to using it?

I used CocoTek

I did rinse every pot a lot, then I loosened them with my hands before planting. How can I maintain "soil" pH in coco coir?

you are not going to change the PH of the coir. My thing was salt build up and then a lack of calmag which most people see. Test the run off next time you water see how many PPM's it is. When mine started to get 700-800 i started having problems. Then i would flush and hit them w/ straight water a couple times and felt like they would starve. So you may try the flush until it gets close to PPM of the tap water then light feed next time. If i had coco to do over again i would try more organic type nutes(fish ferts, guano's, kelps, meals). Not have to worry so much about feeding them concetrate more on watering.