Brown spots on leaves

First signs of a potassium deficiency. Easily solved by adding a P/K booster to your line up, or upping the dosage if already using one.

Low K - the distinctive progression around the serated tips. Barring ph or nute lockout, raise your levels of K a bit while minimizing effect on other nutrients. See if that clears up the interveinal chlorosis too. If it's slowly progressing, always better to tackle one thing at a time unless absolutely certain.

Curling tips and browning edges, too much phosphorus...go easy on the nutrition 'cause you are just begging the flowering. Try to correct doing a couple of waterings with some kelp solution (Ascophyllum nodosum) mixed with water to correct the nutrition excess and after that use in just one day, some Cal/Mag and everything's going to be fine at last. Keep the good work. 420.