Top of plant only signs of Potassium Deficiency

Yes I would say magnesium or nitrogen. It is definitely not potassium!

that looks like magnesium deficiency to me, probably from the lights being too close or the temperature sending the plant into overdrive meaning it needs more Mg. You can tell from the taco-ing on the leafs and the discoloration going from the leaf veins outwards, for K def what you would see is colorful splotchiness . Seems like you can go a bit less on the nutrients in general as well. So cal mag, a chelation agent like humic acid, and keep doing what you're doing with the NPK and the problem will stop in its tracks and the plant completely recovers in 10 days more or less. Hope this helps ! 🚀

Potassium is a highly mobile element in the plant and is translocated from the older to younger tissue. Consequently, potassium deficiency symptoms usually occur first on the lower leaves of the plant and progress toward the top as the severity of the deficiency increases, so I would rule that out. It does show classic signs of having a magnesium deficiency. I did notice you are growing organically so down below is solution to fix the problem without using chemical nutrients. First, you will need to mix together the below in a Chapin sprayer: 1-gallon water 1 tbsp. Epsom salt 2 to 6 mL ThermX 70 (optional) Depending on the size of your garden, you may find you need to double or even triple the above inputs. Prior to spraying your plants, please remember that you want to spray when you are able to turn the lights off immediately after spraying. If you cannot turn the lights off immediately after spraying, you will inevitably burn your plants and weep on the floor of the grow you once had when you return to check on them. And, we don’t want that. The easiest time is a few minutes before the lights go off for the day. Then there is no disturbing or changing the light cycle, just a light rain before the sun goes down. Alright, now that you have found the optimal time to spray them, spray the plants with the solution you stirred up! You will want to be sure to cover as much of the plant as possible with the solution. Let the plants do the hard work from here. After a few days, if you see small improvements, but not much, you can reapply after seven days and the plants should begin to bounce back at this point if they haven’t already. Hopefully, this helps and remember ..."Happy Growing" and "Free your mind one puff at a time".

Hey friend, So if i was gonna give an educated guess....it would be definitely a magnesium deficiency. In the last photo you can really see the stripped leaves, that bright green/yellow in between the veins is usually associated with a magnesium issue. However even though it clearly looks like needing a calmag supplement...it could be triggered by other things. Wrong ph, watering practices, npk too high and locking out. Personally when i started using a regular calmag supplement my leaves were much healthier and happier. Goodluck with the problem solving. ✌️