Which of the following is NOT a type of sand filter used in pools?

A fiberglass filter is not classified as a type of sand filter used in pools. Sand filters specifically use sand as the primary media for trapping debris and impurities in pool water. The primary function of these filters is to clean water by allowing it to pass through the sand, where particles are trapped.

Diatomaceous earth filters and cartridge filters are two other types of filtration systems used in pools, each utilizing different media for water purification. Diatomaceous earth filters use a powdery substance made from fossilized algae, and cartridge filters employ a filter cartridge made of polyester or similar materials. Unlike these filters, which serve similar filtration purposes but are based on different principles, fiberglass filters are more commonly associated with structural components of pools or as part of the shell of a filter rather than as a standalone filtration method. Thus, identifying fiberglass filters as not being a sand filter helps clarify the categorization of pool filtration systems.