I did notice where you said what sort of mixture of fertilizer you use, and I believe how often you use it was also mentioned, but unless I missed it I did not see what type of fertilizer(s) you are using. It might help to mention that if you did not mention it so far.
　
The statement of some pictures looking like nute burn was what I also thought so that is why I asked what type of fertilizer(s) you may be using. As I am sure you know some strains are very forgiving to differing solution mixtures of fertilizers and others are not at all forgiving and depending on what someone uses even a weak solution if given to often can result in burning. Luckily there are not many such strains but it can happen.
　
The comments about checking your pH and adjusting it if needed are very valid but be sure to also check the pH level of your water. You may have started out with a pH range that was within an proper range and you may not have fed your plants enough fertilizer(s) to alter than range enough to cause lockout but if your water is way off the scale one way or the other that could be your problem, or at least some of it.
　
I would suggest giving up on the idea of watering on a set schedule and a set amount of water per watering. Depending on ever changing conditions your plant’s moisture needs will change. Also as I am sure you know plants will require more moisture as their growth progresses so if you continue to give them a set amount before long you will not be giving them enough moisture.
　
Water meters are fine if you pick a quality one but the simplest thing to do is to stick your finger into your soil, about two inches, and if it does not feel moist, or at least cool, then it is time to water.
　
While I admit it is a pain in the butt to do I like to water until there is a fair bit of runoff built up in my drip trays. Then I will wait an hour or two and check to see if the water has remained or if it has been drawn up by the soil. If much water remains I either suck it out with a turkey baster or remove the pot from the drip tray and pour off the excess.
　
When someone uses a soil with a major amount of drainage, which is good, or if they grow in pots that are to small and the soil is compacted by root growth, that is also usually in a near-root-bound condition to an actual root-bound condition, much of the water the plants are given will run through the soil and not be absorbed.
In some cases when someone uses a soil with inadequate drainage over time between root growth and the water itself working its way down through the soil channels or tunnels or whatever you would prefer to call them will be created and again much of the water the plants are given will flow through instead of being absorbed by the soil.
　
Many growers see the runoff and take it as a sign that their soil is fully saturated and that they had given their plants an adequate or even greater amount of water than needed but that will not be the case. Much of the soil will remain dry to mostly dry.
　
By allowing your pots to sit in some water for a short while, doing so will then allow your plants to take in needed moisture from the soil and then the soil will wick up moisture from the drip trays. That gives your plants enough water and also allows your soil to draw up and retain as much moisture as it is capable of holding. By allowing your pots to sit in a small amount of water for only a fairly short period of time will not be risking root rot.
　
Also as was said, I believe several times, more details would be helpful for anyone to best be able to assist you. Sometimes details that someone believes to be so minor or disconnected as to be inconsequential and therefore not worth mentioning can at times hold the key to whatever problem the grower is facing.
　
The greater the detail someone goes into the greater the chance to receive accurate helpful advice. An example might be when someone describes a current leaf condition it would be helpful to explain the very first spotted symptom. Where it was first spotted? What if any progression did it take? If there is and always was overall per leaf discoloration or did it begin at some location on the leaves and then spread? Did it begin as necrotic spotting instead of a gradual overall leaf change of color and then spread to the entire leaf? Whatever the discoloration is or necrotic spotting is, is it between the veins in the leaves and the veins have remained dark? Is there any other discoloration elsewhere on the plant, as in branches and or stems? And of course what color, if any, is it. Even with pictures at times some pictures are not exactly professional quality high definition pictures and are somewhat lacking. They can also most time’s only show the leaves themselves when a shot or several shots of the nearby branching and stem may be very helpful to see.
　
You mentioned your grow room temperature range. Was that at the top of your plants or a mid-room level average? Temperatures should be taken at the top of your canopy. Temperatures in the mid to upper 70’s range are optimal. Anything up to 85 degrees is acceptable. Temperatures above 85 degrees will cause THC to degrade. The plants may seem to be thriving in such temperatures but you will suffer a loss of overall THC production/accumulation. I doubt that is something you desire.
　
Good luck!
