What deficiencies do my plants have?

so we have two problems here, one minor one major, but first let me give you a little idea on my grow.. I am using 4 23w 6500k cfl's which produce 1450 lumens each, and 4 34w 4100k fluorescent tubes producing 2650 lumens each in a grow space of 6 square feet. im using jacks classic 20-20-20 fertilizer, regular tap water which i know isnt the best, and i foliar spray with a magnesium and calcium solution. the Ph is about 6.5. I use an organic mix in 5 gallon pots. temperature is at a constant 76 degrees. now lets get on to the plants! my oldest plants (50 days veg) leaves started turning almost completely yellow and developed brown/black spots, and its moving up the plant quick. growth might have been stunted considering its taking forever for these new sets of leaves to open up. i know it isnt nutrient lock out because the Ph is about 6.5 for all plants and the other two are fine (my third one i hadn't mentioned since it was doing good) plus i fertilize them all at the same rate and same amount so i dont think my oldest one could have too much nutes. From looking around online it SEEMS to be a potassium deficiency, but the fertilizer i am using contains 20% soluble potash which if im not mistaken is potassium. ill post some pics and see what you guys think, now on to the more minor problem but i'd like some info on it now in case it it turns into something big, the new leaves opening up on my second oldest plant (41 days veg) appear to have long brown tips and they almost look dead. i havent found anything online on this so i was hoping you guys would be able to diagnose it... The first three pictures will be my oldest plant, the one with the big problem, and the second two will be of my second oldest with the minor problem.
View attachment 2498328View attachment 2498329View attachment 2498330 View attachment 2498332View attachment 2498333