As delivered, the water tanks on our new boat were in a pretty sorry state. They clogged pretty much instantly the water pump filter with black stuff which I can only assume is mold.
We cycled two tanks forward (87 gallons) and aft tanks (55 gallons) through and cleaned the filter. I then added a cup of chlorox in the forward tank and 5/8 cup in the rear and let it sit. Then drain both tanks, and fill and dump and refill. The water seems reasonable right now, we'll test it at the weekend and report back.