New Growers Yellowing leaves

I don’t see cal/mag listed on your nutrients. As you are growing in coco it’s usually a critical part of your grow. Once we get a picture of the whole plant the full diagnosis will be easier though. Seeing exactly how the plant looks and exactly where the worst damage is tells a lot about the cause. It’s one of those a picture being worth a thousand words things.

Flush it real good with about 7.5ml cal mag per gal of water.  Willing to bet that would fix her up.  More info would help narrow down, but in the meantime flush now

Hey mate, as others have mentioned a whole plant picture would be best to help diagnose a problem. You listed feed frequency as every other day? with coco it needs to be daily feeds minimum, if you don't have any calmag added to your feed I highly recommend adding some asap as coco will draw the nutrients if it isn't buffered with cal mag, which brings me to the most probable issue would be a lockout. Check the ec & ph of your run off, it will most likely be out of wack. Give them a really good flush with ph balanced water with a dose of calmag mixed in then feed the following day with half strength feed and build back up to where you were at. A flush wont hurt the plants but it may save further damage from taking hold. But if you can post a full plant pic so we can see what's going on. 

hey thanks for replies. 
as you can tell I’m very much a beginner. 
Ive been watering when they feel dry. I’ve attached a pic of the the feeding measurements I’ve been going by.

have added cal mag now to the feed. 

Posted by: @zamsam

Ive been watering when they feel dry.

This could be the cause of your issues. Coco should be kept damp at all time and not dry out. That cause a salt build up and can cause problems like this one your plants. 

Make sure the coco doesnt dry out, feed the plant every day and make sure you get atleast 10% run off every time you feed them too. Doing this is a good way to keep a plant happy when growing in coco 

Looks like you are beginning to see lockout. I’m in agreement with Macky, the watering schedule you are on is going to give you grief. I’d do a full flush followed immediately by feed at 1/4 manufacturers recommended rate, like BubbleHok said. Make sure your flush is properly ph adjusted to ph 5.5 - 6.0 and you should see her stabilize shortly.
Here’s a guide to flushing in coco

https://percysgrowroom.com/flushing-a-cannabis-plant/#Flushing-a-Cannabis-Plant-in-Coco

After flushing change your watering schedule to every day for growing in coco and always allow runoff, you can’t overwater coco so don’t worry about that. As Macky said, all this will help prevent the salts from building up again. From the looks of the plant she might benefit from more light intensity. The upper canopy looks a little stretched. Any idea what PAR level you are currently at?

Thank thank you. There is so much info out there and I think I’ve gotten a little bamboozled by it all. I have flushed now with the suggested cal/mag and Ph at5.9. I have now fed with a 1/4 dose of feed and will see what happens. It says the PAR is 1100 but have had light on 400w so will increase to 600w. I will make sure I water daily now. Have been paranoid about overwatering. Thanks again peeps. All advice appreciated.