Not a judgement on this image, as sometimes I would want them as part of the image, but other times would like to not have them… I read it somewhere on what causes this, but can’t for the life of me find that info again…
Those are caused by bubbles. You prevent them by constant agitation–
attaching an aquarium pump to the side of the contained for the vibration,
using a pump to blow bubbles into the mordant to move it around, using a
magnetic stirrer, standing there and brushing off the bubbles with a
feather, etc.