I sometimes see floating dark spots. Vision is normal 6/12 with cylindrical lenses for correction. What is the cause? Any action needed?

Eye exam. Best to have any new finding checked out. A few moving dark spots typically are vitreous floaters, but could have an eye exam to rule out any retinal problems.
Floaters . Vitreous floaters. If this is a new condition it may be the sign of a vitreous separation which should be examined by an ophthalmogist. It could also be associated with retinal tears that can lead to retinal detachments.