You might find it helpful to figure out at what step the problem is introduced.
First just eliminate all steps related to image capture, IE everything before development. No camera, no lens, no film holders. Just the film.
Also, eliminate the tank. Don't use the tank at first: see if the problem lies with the film itself, or how you handle the film. If the problem still appears with tray development, you know it's not the tank, it's the film or your handling of the film.
If the problem disappears with tray development, you know the problem appears later in the process: how you put the film into the tank, the tank itself, etc.
Best of luck
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