From what I am seeing here it does look like a phosphorous deficiency but having recently fixed this situation I am inclined to to go with heat stress, phototoxic burn from insecticides, and overwatering. I have to advise you against keeping dead/dying leaves on the plant.....they are invitations to a host of parasites and fall into the soil and rot on the surface. Over-watering is easy to do with soil and when it shows up visibly the tendency is to fix it with nutrients in the roots only making the problem worse. If you want to get some aeration back beneath the surface one option is to use a long skewer and poke holes gently until you reach the bottom and also give them a few pokes from the holes on the pot going upward..... do not overdo it and break roots though. In this situation, foliar feeding is the way to go. Liquid light by dutch masters is great...you dont have even turn the lights off. Overwatering will always be followed by deficiency if you are in the range you want to be in so add your nutrients to the leaves directly and use a wetting agent like saturator or coco-wet. Btw 1/2 strength on the liquid light just in case. It is all connected....1/2 of your success will be the environment you create. Remember pyrethrins, neem, and oil based insecticides need to dry well before the lights go back on, and raising the light for a couple hours might be a a good idea.