biOrb filter
Water is circulated around the biOrb and passed through several filters. As water passes through the rocks at the bottom good bacteria remove the toxic elements of fish waste. The physical waste and debris are then drawn into and trapped in the filter cartridge where excess pollutants are also absorbed before the water passes out back into the aquarium. Unlike traditional filters the biOrb filter cartridge can be completely removed or cleaned without affecting the good bacteria which always stay in the aquarium.
External canister filter
These filters are units which sit outside the aquarium. Inside the unit is usually a combination of biological, mechanical and chemical filtration. The water passes out of the aquarium, into the canister and then back into the aquarium. External canister filters are usually used in very large aquarium set ups or those where disturbing the decor inside the aquarium would be difficult. The canister needs to be regularly cleaned and different elements replaced at different times, whilst they are very effective filters they can be complicated and time consuming and therefore perhaps not ideal for inexperienced or time-poor fish keepers.
Internal filter
Internal filters are boxes which sit inside the aquarium, usually near the water surface. Aquarium water is drawn into the box where it passes through different types of filter media to trap dirt and remove toxins before passing back into the aquarium. These boxes can need careful cleaning as they usually contain the beneficial bacteria for biological filtration and cleaning them too well or replacing too much media can cause serious aquarium problems. As dirt sinks to the bottom and these filters sit at the top they aren't always effective at trapping dirt.
Under-gravel filter
As the name suggests a large perforated tray sits underneath the gravel and creates a space where the water can be filtered. Water passes through the gravel and into the space underneath where it is drawn out through a vertical pipe, either by an air stone or power head. Bacteria in the gravel remove toxic fish waste and physical dirt is trapped in the gravel and under the tray. The trapped dirt needs to be removed by vacuuming the gravel. Periodically the gravel should be removed to clean out the dirt from under the tray.