In a gravity fed filter system the water leaves the pond through a bottom drain and enters the filter along a length of pre-laid rigid pipe. Water is drawn from the filter, which is installed at the same level as the pond, by a pump, and is passed through a UVC, before returning to the pond.The component sequence in this system is;

Pond ? Filter ? Pump ? UVC ? Pond

In principle, a gravity fed filter works in the opposite way to a pump fed filter system, with the pump drawing water way from the filter either via a suction hose, or been situated in the filter's final chamber.Because gravity will always keep the water levels equal, when the pump pulls water from the filter, pond water is replaced in the filter via the bottom drain.Gravity fed systems are generally more efficient when compared to pump fed systems; bottom drains require very little maintenance and are able to remove all pond waste, whereas pump cages can become blocked over time.The pump in this system is pumping clean, filtered water and therefore does not need as much maintenance. It also doesn’t usually need to pump the water to as much of a head in comparison to many pump fed filter systems, so the flow-rate isn’t reduced as much.There are no cables or hoses coming from the pond which can look unsightly and may trap debris in the pond.Water entering the filter isn’t as turbulent as in a pump fed system, which aids the operation of a settlement chamber. And, because the water entering the pond is sent directly from a pump, a venturi can be installed to the outlet which injects air into the returning water.The drawbacks with a gravity fed system are that they are not easy to install. Most pond owners require a qualified professional to construct and install the pond and filter system. This, and the fact that pipe work, etc is required, makes the build more expensive. As they are installed as the pond is built, they cannot easily be altered or updated. Also, the UVC on this system has to be installed after the pump, which isn’t ideal as flocculated algae has to return to the pond before been filtered out.