Pressurised filters:
A pressurised filter is a water-tight canister that utilises the pump flow rate to force the water through a collection of sponges in order to filter out the debris within the pond. These filters can be located either above or below the water level, this gives these filter the ability to be buried in the ground up to the securing strap, perfect for ponds with open space surroundings and not much foliage.
These filters are sealed units and can be used in conjunction with waterfalls and fountains, being pressurised they can still pump against gravity.

Box filters;
Box filters or gravity pond filters rely on gravity for the return of the water from the outlet or bottom drain. They should be installed at the same level or above the pond surface so the water can easily return to the pond. If it is below the surface then the water will back up (causing it to flood), this is because the is no back pressure to force the water uphill. To use a gravity pond filter with a waterfall, it will need to be installed at the top of the waterfall, a suitable pump with sufficient head pressure would need to be used to get the water to the top of the waterfall/ feature.