Leaves are curling up and drying out on one plant. The smallest.

Anyone that can help me out here would be great. I am new to the growing world and with all the information out there I can't find what is going on. Temperatures are between 72-76,humidity 40-50%,canopy temp is about 75. My Ec is around 1.7. Ph is constant 5.8-6.1. I am thinking it is a nutrient issue bit am reluctant to just start guessing. The leaves are curling up and down at the tips. The leaves are drying right out. I don't believe it is the light as I have raised it. The grow cubes are pretty dry by the time the second feedng is taking place. They were only being fed once a day with a drip system. There are two fans lightly blowing on the plants. This one is the furthest away. Air is exchanged every 10-15 minutes.
I am going to try to just water with ph balanced water and epsom salts. To see if it is over fertilized. It is the runt of the litter and may be too small to take what I am giving it.
It looks like a phosphorus deficiency

I'd say, check the root system..
I was gonna go with Potassium deficiency, but I really don't think it's overfed..

Last pic looks like potassium deficiency.

Thanks for your reply. Roots are looking good. I am in the tall 5 gal. buckets and will likely go to the shorter ones. The plants that have their roots in the bottom of the bucket are in way better shape. The 4 inch grow cubes may be gone next time as well. They hold too much moisture I believe. Issues started when I fed them in the evening. Blocks stayed wet for too long. This is my first grow and I have made a couple of mistakes that likely caused slow growth. Some just recovered better. All in all I am just trying to learn and not kill them in the process.
I had also just changed the nutes by adding seaweed fertilizer and cal-max to the Lucas formula (reduced version to keep the ec down) and cannazym . I try to not change too many things at once. The changes did bring on some nice new growth. The blocks just seemed dry and that was why I fed them in the evening. Blocks were still wet the next day. Don't want to jinx myself but for the first grow it is going pretty good.
Is there anything that you would recommend for my nutes? Is once a day feeding normal in a drip system? The plants are looking like they are doing ok. Sort of have to go with the plant I guess?
Thanks again for your time and information.

I will dig into the potassium. I was using ro water but the ppm was around 350-400. I switched to the tap as it was zero or close to it. It is hard water. Thanks for your reply.