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.

Ben used to farm Koi for a living and has published three books on ponds. See his website watergardenersbible.co.uk for more details.