The size of the filter will depend a lot on the bioload or amount of fish waste that needs to be cleaned up. One also has to consider that the fish will grow during their stay in the pond or tank and the bioload exponentially increases as the average length of the fishes increase. Some filters use more efficient filtration media than gravel; for example plastic excelsior ribbons or bioballs. However if one plans to also use bog or hydroponic plants to remediate the waste then gravel may or may not be necessary depending on the plant species. Research has shown that some plant species are much better at removing and retaining waste products than others.
