There are three kinds of pool filters: cartridge, sand and diatomaceous earth. While there are different maintenance procedures for each type, all require periodic cleaning depending on the type of filter and how often a pool is used. Cleaning the filter more often than recommended can actually hinder the filtration process. A clean filter is less efficient than one with a mild amount of dirt in it because the dirt helps trap other particles, which removes debris from the water. However, you don't want to let the filter get too dirty. A sign that it's time to clean is an increase in flow between the pressure gauge and flow meter. Clean the filter when the difference reaches 10 to 15 pounds (4.5 to 6.8 kilograms) per square inch.