What is it? Overwatering, nutrient burn or deficiency?

no light = dead leaf solution = lollipopping 💚

The plant puts in the lower leaves that it is ready to sacrifice toxic elements or overdosed elements. Mineral fertilizers can cause this effect by forcing the plant to assimilate them even if it does not need them. It's normal so, there's no need to panic, the plant protects itself and manages the elements it accepts and those it rejects. 😉✌️

The 'red' i see may also be a camera illusion.. if you don't see reds or purples with your eye, nevermind.

overwatering does not cause deficiency/toxicity symptoms. that is due to a lack or over-abundance of nutrients around the roots. Image search for a 'marijuana leaf symptom chart' add 'cervantes' to get the one i prefer near top of list. your tastes may vary, but should be able to find a 4k image you can zoom in on for great detail. This is very overtly a N deficiency. Increase proportion of N to your fertilization process while mnimizing impact on other nutes as best you can -- i.e. the bottle with the highest N and least amount of anythign else. That will solve the paling starting at the bottom and creeping upward. Adjust slowly so you don't go too far the other way, and don't expect things to fix themselves after 24hours.. necrotic damage is lost, but mild paling will get green again... make sure it doesn't get too dark after the adjustment over time. Pic 4 look like some red/purple on the leaf? this is due to genetics or cold temps (68F or below). It's the same reason leaves change color outside in Autumn too. Usually not an issue. if it's early in flower, i'd double check night time temps and maybe employ a space heater if it's dropping into mid 60s or below.

I wouldn't freak out or change anything just from a couple yellowing leaves hidden under your canopy. Judge the whole plant. That kind of yellowing is very common in flower and really isn't an indication on how your plant is performing. Plants will self prune extremely shaded leaves because they use more energy than they create and they can also cannibalize those leaves for nutrients. Now if you get more yellowing throughout your plant well before your flowers finish, THAT would be more concerning.

Not enough direct light 🌱💚

Based on your diary entries I noticed this was the first week you didn’t include the “grow” nutrient which I’m assuming is your main source on nitrogen. Looks like a nitrogen deficiency to me as well

I put here the answer that i just give you 4 days ago. I add that you need a flush now and then you must adjust your feeding with my past tips. Hello mate, you have an issue with Ca/Mg caused by a pH fluctuation or a misfeeding of Calmag product. The first cause is often link to the rate of Ca/Mg in the water used to watering the plant, if you use organic feeding you must have the rigth rate of these nutrient that is 3/1 or 4/1. Good luck and happy journey!

Looks overall just hungry maybe be a few deficiencies starting and needs some nitrogen for sure. I would give a balanced feed they are at their most humgry point in life

dechlorinate your water and increase nitrogen and