I recently had some renovation work done and my pool had to be pumped out into holding pools. While the work was being done in the deep end a bit of water leaked into the shallow end from the steps. The water was reddish brown presumably from the ground water under the liner and behind the steps. It pooled (maybe a few gallons total) and I assumed when the pool was filled up again it would all wash away. However, it stained. I got my chlorine up high for a couple of weeks (7-10 ppm) and brushed it hard to no avail. Any ideas how to get rid of these mud stains?