FILL WATER:  Used in filling or adding to the water level.  Whether from the hose or from a well, your fill water brings its own chemical make up and water balance (or lack thereof).

FILTER:  A device used to remove particles suspended in the water by pumping water through a porous substance or material.

FILTER ELEMENT:  A device inside a filter tank designed to entrap solids and direct water through a manifold system to exit the filter.  Cartridge filter elements and DE filter grids are two examples.

FILTER MEDIUM:  A finely graded material such as sand, diatomaceous earth, polyester fabric, or anthracite coal that removes suspended particles from water passing through it.

FILTER PUMP:  The device that pulls water from the pool and pushes it through the filter on its way back to the pool.

FILTRATION RATE:  The rate of water pumped through a filter, in gallons per minute (gpm).  

FILM-X:  A compound of citric acid used in cleaning plaster and other pool areas. Safe replacement for muriatic acid.

FOAMING: A term used to describe surface foam on your water, esp. in spas/hot tubs. Foaming is caused by high TDS levels working in combination with soft water and oils. Certain low grade algaecides can foam when added to pool or spa. Use enzymes for foam control.

FLOCCULENT: Essentially the same as a coagulant, this chemical (such as alum) is used to combined suspended alkaline material and/or algae into a heavy gel, which sinks to the bottom for vacuuming to waste.

FLOW RATE: The quantity of water flowing past a specific point in a specified time (e.g. the number of litres flowing through the filter in 1 hour).