Cannabis plants can reach sexual maturity at 4-5 weeks old. Doesn’t mean you’ll ever see flowers in a month. The transition to flower time is 100% dependent on the photoperiod/s it was crossed with. After a plant has reached sexual maturity it will still take several weeks to begin showing early bud sets.

Is there any media under those rocks or is it 100% hydro? If so, what’s the water temperature? About 68-70F is where you want the water temperature with hydro.

Not all leaf issues are caused by a lack of available nutrition. Especially mineralized bottled or powdered nutrition that contains all the essential elements. Mineral lockout and mineral unavailability are two different things. There are many ways to damage roots.

Necrotic lower leaves are typically caused by pH stress (hopefully) or pythium (root rot).

Symptoms will always get worse before getting better with damaged roots. The time it takes to recover during flower severely affects yield/quality.