Deficiency of some sort. Could be nute lockout. Soil pH needs to be 6.5 [6.3-6.8]. Or it could be nute burn (too much nutrient food). Do you have a soil pH meter? Do you pH your water before giving it to your plant?
your doing alright, doesnt look too bad. do you have a PPM meter? they prob just need to be fed, i keep my ppm 800-1000, but a lot of people bump up to 1500+, just keep it simple, its a learning process, youll get it!
its hungry and thirsty. Your plant is healthy and vigorous so you can water and feed more than you are, dont let the pot dry so much and it will greenup.
What are you watering with RO? or tap sometimes your tap already contains enough CalMag no need to supplement that, the leaves that dont get enough lite tend to whiter and die off yellowing like that is could also be indication of N def. What type of soil are you using? What nutes if any are you using? What type of water are you using?
Don't worry about some of the bottom leaves yellowing and falling off. Plants are very industrious. As a plant matures, especially under artificial lighting, it will naturally shed leaves which it deems too inefficient for photosynthesis. Light intensity to the the lower regions of the plant quickly degrades as the light source is raised to accommodate new canopy growth. By dropping the bottom leaves, the plant can then distribute that saved energy to more vital areas.