Pool service involves a lot more than just skimming debris out of the water. It includes testing the chemical levels and adding chemicals as needed to prevent bacteria growth. It includes cleaning out all baskets that are built to catch debris. Cleaning the filter cartridges to ensure the right amount of water pressure is able to get through is an additional cost. It also includes cleaning the top of the water by skimming the swimming pool and then cleaning the bottom by either brushing or vacuuming. A final step in cleaning involves backwashing, or using the water from your pool to rinse dirt and debris from the filter.