YELLOW LEAVES

if it's only on the 3-fingered older leaves at bottom, i'd chalk that up to just a blemish. Those early leaves often have imperfections and i don't see any "symptoms" on the 5 and 7 fingered leaves on any of the other plants? you'd know better than me. What i do see is some hints of interveinal chlorosis that probably have nothing to do with the blemish you see in the first picture provided here. a little leaf curl, so total mix +soil amendments may be a bit high. lockout and pH-related t hings usually have multiple symptoms of differing origins, which i don't see here, but somethign you should look out for if my suggestion doesn't fix it. pH is a concern if you don't have any clue what it is. it is important to eliminate possiblities. i don't see anything crazy going on and soil does compensate a good bit for you on its own. i'd still spot check and make sure nothing crazy is occuring -- your fertilized water, the tap water you use (or filtered?) and maybe this time and in future if oyu have a problem the runoff / soil slurry pH. Once you have an idea, you don't need to test constantly. i only test before i germinate seeds to avoid killing them. it'll take same amount of acid or base to change the pH each time, if you have to. spot checking after you measure that out is fine. i don't even have a pH pen. i buy a 100-pack of strips every 2-3+ years for 5 USD. because i don't see other coinciding issues, i'd lean toward upping the Mg ppm 10% and see what happens. i see you are in soil, that becomes less precise adjustment... take the labels of your fertilizer and figure out about 10-15ppm added to Mg. i provide roughly 85ppm + my very hard water adds some too... 10-15ppm is about a 10% increase... little over that obviously. As with all advice, you need to consider what info was not given... and do a little homework to confirm what was said to you... repeat it enough and you learn quickly. get a leaf chart, start studing it each time you see a symptom... eventually it becomes familiar.