It could be some sort of nutrient deficiency, but not sure which one. When I clone, the bigger fan leaves usually take a hit and start looking real crappy. I just usually wait for a good amount of new growth to take place and I just pinch the nasty fan leaves off, typically by that time they are already half dead anyway. You mentioned that your PH changes daily, going from 5.5 up to 6.3. I kinda feel like that is probably your biggest problem. I ran into the same problem with my PH jumping so fast, having my PH go from 5.8 to around 7 within 24 hours. It did it every single day until I just gave up and chopped before they had a chance to produce anything worthwhile, and I wasted so much of my PH down solution just to keep it in check. If I remember correctly, I think mine was due to slime building up in the bubble buckets I was running. It was the first time I ran bubble buckets, and I used those all black ones, which in my opinion are bad. The reason I say they are bad is because dark colors retain heat instead of disperse it. So, since the buckets were black, and being under two 600 watt HPS lights the bucket would transfer the heat to the water, making the water temps rise up to around 78-80, which was a breeding ground for bad bacteria. I was rinsing the roots and cleaning the buckets daily, too much hassle.
I don't know for sure if this is what caused my PH to raise every day, but with the water temps being too high it made the roots get covered in a clearish slime. I feel that the slime that built up overnight is what caused the PH to jump. It's the only thing that I could think of that was a constant factor in the problem. I could have proven this to be kinda true if I would have just transplanted them into my NFT system, which I already had broken all the way down and stored away, and see if the slime would return, but only after rinsing the roots really well and give them a good soak in some diluted H2O2. The NFT system I ran never had problems with water temps, I built it out of white PVC and a big blue tote, which I ended up spray painting white to also help bounce the heat off of it.
You didn't mention anything about having slime on your roots, so I assume your roots are fine. I know I just went through that long story and probably didn't help you solve your problem. The only thing I can say is keep an eye on your water temps, as well as your roots. My PH problems didn't start to happen until about a few days before the slime appeared, so I just assumed that was the problem. Once I went back to my NFT system, and the water temps was staying in the right range I didn't run into the slime problem, or the PH problem again, maybe it's a coincidence, maybe not.
