Consistent water quality makes for healthy, happy fish.
This requires proper filtration, and here are the three main types:
(1) Mechanical filtration removes particulate matter. The goal here is crystal clear water (accomplished by fine-mesh filter).
(2) Chemical filtration removes toxins. The goal here is to remove chemical pollutants (accomplished by carbon filter).
(3) Biological filtration cultivates beneficial bacteria that consume waste product from fish and other animal life. The goal here is to mimic the process occurring in nature (accomplished by oxygenated water in contact with aerobic bacteria).
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Undergravel filters marked the birth of the home aquarium. Basic principle of biological filtration is oxygenated water flowing across media. Plants consume the nitrates.
A common wet biological-filtration option.
Wet-dry configurations are highly customizable. Modern media offer more surface area than gravel or foam. Chemical (e.g., carbon) filtration here is added downstream from the bio filtration.
Wet-dry filtration system increases media (in this case, foam) contact with oxygen, which allows for larger colonies of beneficial bacteria.
Overflow box at top skims surface contaminants and prevents catastrophic water-loss if a pipe leaks.
Protein skimmer removes excess organic-compounds, and baffles maintain ideal water-level for this operation.
Saltwater setups often include exotic apparatus like protein skimmers, and UV sterilization chambers for eradicating microorganisms.