yellow discoloration and spots!!

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kay so ive been growing two willy nelson clones sativas for about a month or so. They are both around 25 inches tall. they are under 6 100 watt cfl's and the average temp inside the grow box is around 75 degrees. they are recieving about 17 hours of light and are about 6 inches away from the light. i water them every couple days when i feel they need it. i gave them some weird nutrient solution i got from my cuz (just a tiny bit in a gallon jug of water and the tips turned brown and spots started to appear on the outside ridge of the leaves. i figured the nutes fucked them up so i just flushed them a couple days ago to get the nutes out. i mist them on a daily basis keeping the humidity around 50 to 60%. lately ive noticed that a few leaves (some on the bottom, in the middle and on top) basically a few leaves here and there all over are turning (slightly) yellow and there are a few tiny brown spots that seem to appear. the plants seem overall healthy but im just concerned. can somebody help me diagnose this mysterious problem??
 
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