Koi

 Ben Helm says...

These two terms are taken from a filter's perspective. A pump-fed filter is probably the most widely used in smaller ponds. Driven by a submersible pump situated within the pond, the filter is situated outside and above the pond as literally pump fed water from the pond under pressure. Typically, the water within a pump-fed filter will trickle down through the media (although sealed, pressurised units are available) and return to the pond by gravity, usually feeding a waterfall. A gravity-fed filter is situated adjacent to the pond, at the same level. Fed by a bottom drain, a gravity-fed filter will usually be multi-chambered, with the final chamber housing the pump, which returns the water under pressure to the pond (often via a venturi). Consequently, the pond's surface will always be slightly higher than that in the filter, making water pass through to the filter via the bottom drain under gravity.