URGEnt - leaves turning yellow

This looks like classic N deficiency, if its being caused by a lack of it or PH issue is the question. I don't think you need a flush I think you need to look at your inputs and make a informed decision. We don't know what your feeding them with at 0.7EC, normally this could be ok, its on the lower side of feeding for a photoperiod. but if all your feed has in it is P and K and no N then that's the issue. If your growing and soil and using dry amendments the microbes tend to keep everything in PH range unless something is really off in your medium. If your growing in soilless or hydro style the nutrients provided are available from the wide PH range of 5.8-7. I like to keep mine aorund 6.2. Gives me wiggle room up and down on the scales should something go off. Coir is also special in that it need buffering with extra Ca and Mg. Go look at your NPK labels and and see what you are feeding them. combine all products being used. You can no longer reply to questions so if you have questions use the DM's or ask another Question. Good Luck!

.7 EC for that plant should not be an issue. My concern is it looks like you have nutrient lockout due to pH imbalance. You should be targeting a pH in soil of roughly 6.2-6.5 for every feed and water. Our plants have a particular pH range where they can uptake nutrient. There is a helpful sheet someone did that shows all the nutrients and their pH absorption range. When the pH goes out of range they cannot up taken up by the plant. I suggest that if you do not already have a pH reader that you get one asap. Then you can flush out every pot you have with a pH adjusted water to correct the pH of the soil. When doing this I like to use roughly 3-4 gallons of water in a 5 gallon pot. Most of the time I take the plant to a mop sink for easy cleanup I hope this helps.