hornday's u/s concentrated solution mixed with distilled water - i keep meaning to experiment with some of the online u/s solutions.
the thing for me about the u/s is that it's quiet and fast and does as good if not better a job than my rebel wet tumbler with s/s pins. i see no reason to back to wet tumbling, and i'll only use dry tumbling (with cover off) to dry off the brass after using the u/s machine.