Locked out from nutrient buildup/salting of the medium.
Can you flush those buckets with plain fresh tap water- is there a way to do that with Hempys? Then re-fill them at pH 6.0-6.2 and go back to your normal routine.
The few VERY yellow low leaves are just from lack of light and do not indicate an issue. The burnt tips and other pale leaves with dark veins say 'lockout'.