Bottom drain gravity fed filter and pump
A pond with a bottom drain or gravity fed filter system must be designed at the outset to use this method and have a bottom drain installed. They are more complicated to install but worth it if you have something like a large koi pond. The basic principle is that the water leaves the pond via a bottom drain at the bottom of the pond (hence the name bottom drain?) which leads downwards underground and then up to a filter where the water is filtered before returning to the pond via a pump at the end of the filter area.
Having said this although that is the basic principle it actually works the other way round, i.e. the pump pumps water from the filter area back into the pond which intern causes the water to be pulled through the bottom drain so the levels on both pools of water remain level. This is why these need to be carefully constructed so that this flow process works correctly.
Constructing a gravity fed bottom drain pond filtration system
The theory is fairly simple really you basically have two areas of water, the pond and the filter whose water levels need be parallel. You then have a pipe from the ponds bottom drain sloping downwards underneath the filter, then up and into the top of the filter.
When water is removed from the filter into the pond via the pump, rather than the water level in the filter going down, and the pond water level going up, the water levels will actually naturally level themselves via the bottom drain system (well it will if it is constructed correctly :-).