One bad leaf does not a deficiency make my friend.
Not even that bad. Maybe you slopped something on it or it's just not as happy as all the other leaves and wants a hug.
When you have a real deficiency all the leaves of the same class will begin showing the same symptoms. Older fan leaves act up when some mobile nute is missing but the rest of the plant looks great and may be growing really fast. That fast growing is what's causing the lack of that nute as the plant can steal it to use up top where it needs it more if it can't get it quick enough thru it's roots or it's not there to be had so needs feeding.
The mobile nutes are N-P-K, zinc and magnesium. All the rest including the all important Calcium are immobile. In other words if the plant can't get it from the roots it can't steal from anywhere else so the newest growth starts to suffer first while the older parts of the plants look just fine. All the grow tips will show the same kind of symptoms. Twisting, going yellow, distorting, leaf stems going dark purple, spot etc.
In many if not most cases it's not a lack of micro-nutrients it's that the pH has gone too far off the mark and usually too high. This locks out those nutes so the plant can't use them. With tap water almost all the minerals in are still in the pot when the water is gone and each time it's watered the minerals add up like the scale inside a kettle. Extra runoff should be done each watering to keep that from getting too bad. The harder the water the more runoff you need. Another good reason to use RO or distilled water. It's a blank slate you build on.
They're looking great to me so I'd relax and enjoy the grow.
