Reverse flow undergravel filters are slightly more efficient since they take water from the top of the aquarium which contains more oxygen. Many kinds, sizes, and shapes of trickle filters have been in use since then. Mechanical - Mechanical filtration refers to the removal of particulate matter from the aquarium. Canisters with pleated cartridges and power filters which force the water through some kind of floss are the examples of mechanical filtration. Undergravel filters also act as mechanical filters by trapping debris in the gravel.
Mechanical filtration is good for keeping the water clear and free of debris.
It should be mentioned that diatomaceous earth filters, commonly available in canister filters, can remove particles as small as 3 microns in size which really 'polishes' the water. They can be used remove bacteria and algae blooms but generally clog after a short period of time and should not be used on a continuous basis. Chemical - Chemical filters are used to remove things that are dissolved in the water, and therefore cannot be removed by mechanical filters. They are most useful in getting rid of the yellow color that often develops in aquarium water over time. The most common chemical filter is activated carbon, used mostly in canister and power filters, it traps many substances in it's minute pores.