Yellow spots on bottom leaves.

bottom only? meh... be more worried if it spreads. do watch how it progresses in case it does spread as that helps diagnose with more confidence in conclusions. leaves have a life cycle. the plant does shed them at times on their own. Older leaves aren't as efficient at photosynthesis but still provide other benefits, so let the plant choose what to shed. These are shielded and older leaves. Could be a sign of slightly offbalanced feed, but if it doesn't spread no big deal. if you don't have a good wet dry cycle and good irrigation habits, this could be damage from prolonged bad habits. let top 1" dry in soil. let top of coco start to change color before re-irrigating. maybe drop biggest contributor of N by 5-10%. use all the info at your fingertips and choose one... take notes... inhindsight you'll learn if you did the right thing and if not you still learn something. N tox causes dark green and glossy leaves, which may be the case above the ones you refer to in this question. the leaves at the bottom are not N-tox.