My initial guess is that it’s a potassium deficiency. But I’m just looking at leaves here. Can we get a picture of the entire plant? Did the yellowing start near the bottom of the plant? Everywhere at once?
There’s smarter people around that can assist with correcting the issue. Fill out the grow journal checklist that @Slym3r posted, and the nutes nerds can all chime in as well.
But these charts are a great place to look for your initial diagnoses for your plants.