Three strains fun

Did you increase your light intensity recently? I've had similar looking problems occasionally going from my veg tent with low light to my flower tent. I always attributed it to the light intensity and the increased need for magnesium because of LED, but my issues might not be the same as yours. good luck figuring it out.

It is burning from feeding too much and too often - 3 times in one week is waaaay excessive for soil cultivation! In soil it is....plain water/plain water/feed/plain water/plain water/feed........... so that your plants are only being fed every 12 days or so. Autos can also usually get by just fine on half strength nutrients. At this stage I would recommend that you only give plain water (in normal amounts and at normal intervals) for the next 2 weeks, your plants need to use what is already in the soil before any new nutrients ae added. There is no need to "fush" your soil with gallons of water to remove excess fertilizer salts, flushing can actually release too many/even more fertilizer salts all at once and increase the burning greatly. So, give just plain water for the next 10-14 days (14 best) before resuming feeding with half strength nutrients. The burned leaves will not recover, but hopefully the new leaves should be ok or only slightly burned, - as long as you do not add any new nutrients over the next 2 weeks that is! This condition is in no way a sign of any sort of deficiency or excess iron by the way. Hope this helps, Organoman.

What type of water do you use? This looks like iron excess too me.