Hello fellow enthusiasts, I work in a production black and white lab where we process anywhere from 40-60 rolls of customer film at a time. We're dealing with varying brands, speeds, and a healthy mix of 35mm and 120. We have a workflow set up that batches films with the same processing time together in the same tank. Our tanks vary in size from 8oz to 56oz, all stainless steel tanks and reels. Lately, we've been noticing some uneven development on the film coming out of the darkroom, especially in large even midtone areas like skies and sand. We've included a sample image (from a customer roll, 120 6x9) that depicts the phenomenon in question. Our current process is as follows: All at 68 degrees Fahrenheit: 1. Pre-Rinse in H20 5 minutes. Agitating for first 30 seconds, and then 5 seconds every minute. 2. Develop in Xtol (stock) or Rodinal (1:25 or 1:50) Time varies by film. Agitating for first 30 seconds, and then 5 seconds every minute. 3. Water Stop in H20 30 seconds, constant agitation. 4. Fix in Ilford Rapid Fixer (1:4) 5-7 Minutes. Agitating for first 30 seconds, then 5 seconds every 30 seconds. 5. Hypo Clear in Perma Wash 1 minute, constant agitation. 6. Wash in H20 30 Minutes, running water bath. 7. Final Rinse in Photo Flo 200 Dip & Dunk Each roll individually. 8. Air Dry We would love to hear any and all suggestions about what to try to get rid of these marks. If anyone can figure it out, it's the folks around here.