Gavita recommends 18 inches in flowering, I’ve found 20 inches is better and that’s where I flower them at. Any less than that, the planets will react negatively in various ways. You will need to raise the LED’s 20 inches above your tallest plant, and raise the shorter plants by whatever method you can. Or not, but anything less than18-20 inches away will have issues as you get later in flowering. I have not seen foxtailing, but leaves will start to show some distortion and/or look “off”....... and bud development will suffer.
The key to maximizing these lights is having an even level canopy or you’ll be facing the exact issues you have. Your room looks like it’s off to a great start, but I’m guessing the co2 and/or extended veg time has you in the situation you are now finding yourself in. I‘m getting 53 oz average under each of my
Gavita’s, and keeping the plants very short along with strategic defoliation has me getting yield numbers few others are reaching- 2.3 gpw. I hope to do even better with my current crop.
With all that, you need to run the room warmer with LED’s and co2, probably 82-85 degrees is ideal for you. Humidity is also important to have properly set.
Search out my Orange Gasm grow here, as it will give you just about all the info you could probably want answered.........