They older stuff on the bottom and in the middle of my plants always drops off as the plant gets larger and more dense.
Plants seem to know what leaves are non producers and will lose them.
I've done something similar from two 55 gallon vinegar barrels that worked well.
My setup was a 1/6 horse submersible pump with 1/2 barb adapter through a 120 mesh filter, then a 1/2 valve.
The final flow was tweaked with the screw top 1/4 sprayers. Small plants got one sprayer, large got two...
One old bottom leaf is nothing to be worried about. Older shaded leaves will be dropped by a fully healthy plant as it is unproductive.
Post of pic of the middle age growth that is receiving the most sun. Those leaves are the main producers for your plant and what I would be concerned about.
Been there before, cut off the bottom and score the side with a razor knife. Then burry that in 20-25 gallon bag of fresh soil. You must be in a mild area, I have plants that size in 15 gallons and they need water at least once a day. I've noticed as temperatures rise plants respond much better...
This thread is probably for members with more than a couple grey hairs.
Some of the cartoons that where on your favorite channel but will never see that light of day on public TV again.
I remember having the book and sing along album as a kid for this.
If the nutes are in full solution they would move with the water. Evaporation could be a factor with a large thin mat, making the EC higher than the rez.