agree on the air issue if the buckets receive the same feed. Once those roots close off from the toxicity, they hit the stored chlorophyll and start cannibalising. The new growth is also hit as it emerges so hasn't had the chance to get any nutes . air bubbles ideally need to be 'popping" on the surface a little and not just be big bubbles either. it is the disturbance at the surface that is the beneficial oxygen exchange. waterfalls are the golden standard for the best example. hope you sort it