black water holding tank valve opens but won't drain

pianotuna wrote:
The ice does just about nothing--it is the water and the detergent.

When we deep sea fish for tuna we make an ice slurry in the bow compartment and when it starts to melt we can hear the ice cubes sloshing back and forth rubbing against all sides of the aluminum compartment so I have to believe that 30#'s of ice does something, YRMV