Filtration is the mechanical and biological heart of the aquarium. It removes debris, supports the bacteria of the nitrogen cycle, and may include chemical media to polish the water. Common filter types include hang-on-back (HOB) units, which sit on the tank rim and are easy to maintain; canister filters, which sit outside the tank and offer high capacity for larger setups; simple sponge filters driven by air pumps, which provide gentle flow ideal for breeding tanks; internal filters placed inside the aquarium; and sumps, secondary tanks beneath the display that hold equipment for larger systems. Flow rate is usually sized at four to ten times the tank volume per hour to keep water moving and oxygenated.
Filter media falls into three functional categories. Mechanical media, typically sponges or floss, physically trap particles so they can be rinsed out. Biological media, such as ceramic bio-rings or porous sponge, house the bacteria that convert ammonia and nitrite. Chemical media, most commonly activated carbon, adsorbs certain dissolved compounds, odors, and leftover medications after treatment, though it is not always necessary in every tank. Within the filter, biological media should be gently rinsed in old tank water rather than tap water, since chlorine kills the bacteria that make the cycle work.
Heating and substrate round out the core equipment. A heater matched to the tank size maintains a stable warm temperature, verified daily with a thermometer. Substrate can be gravel, sand, or specialized planted substrates, usually laid one to three inches deep; it provides aesthetic appeal, a surface for beneficial bacteria, and a rooting medium for plants. Decor such as rockwork creates structure and territories for fish, while driftwood adds natural aesthetics and can gently lower pH through released tannins. Because fresh driftwood releases these brown organic compounds into the water, it should be soaked or boiled beforehand, and hobbyists who enjoy the tea-stained effect sometimes build blackwater aquariums that mimic Amazonian habitats.