Fading bottom leaves

if it only happens to an old leaf that is mostly shielded at the bottom i'd hold off to see it progress beyond that before i react. Older leaves will eventualyl mottle even if the plant is in great health overall. The seeming start of cholosis is either a deficiency or the plant is shedding that leaf... If it doesn't spread, i wouldn't react. If it creeps up on the plant, you know it is related to a mobil nutrient and you start cross referencing a leaf chart with what was in your last few weeks of fertilization. Make and educated guess and that's all we can ever do. If it does progress post new pics and get a 2nd opinion if unsure. numerous things can cause little blotches of chlorosis. If in soilless context, knowing what you fed (ratios of each and overall concentration) goes a long way for diagnosing problems - makes it almost simple if familiar and you have a good base formula.