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That would be calcium/salt scale and any commercial product like "CLR" or 'LimeAWay" probably would break it down, but you'd need to pour it in, then close the through-hulland add more to let it soak in the pipe.
Vigorous flushing is supposed to minimize that problem, I was told "always pump ten times whether you need it or not" to prevent that. (Can't do that to a holding tank, obviously.<G>)