What does the whole plant look like? Pics?
All green, just bottom leaves looks like this
Is your girl in flower? If so, bottom leaves turning yellow is normal from what I have read on this forum.
Bottom leaves = mobile nutrients… my guess N but like @JaneQP JaneQP said, if it’s late in flower it’s normal if not, you either need N or pH is off and not allowing N uptake.
My guess was also PH, I have tank 2000gallons and if I need PH at least 7.3 I need to spend 100$ for advance Nutrients PH down just to adjust PH