Biological
filtration media.

§01 — What biological media is.

Biological media is a high-surface-area substrate inside a pond filter where nitrifying bacteria establish colonies. Two bacterial groups perform sequential conversions:

• Nitrosomonas and related genera oxidise ammonia (NH₃) to nitrite (NO₂⁻).
• Nitrospira and related genera oxidise nitrite to nitrate (NO₃⁻).

Both colonies require a stable surface, dissolved oxygen, and a continuous flow of pond water carrying the substrates they consume. Biological filtration media exists to maximise the colonisable surface area inside a fixed-volume filter chamber.

§02 — Static plastic matrix.

Static plastic matrix media is supplied as rigid foam-like blocks or tubes. Manufacturer specifications cite per-litre or per-cubic- foot surface area in square metres — for example, 600 m²/m³ or 800 m²/m³. The blocks are stacked or stuffed into the biological chamber of a flow-through filter and remain stationary as water passes through.

Common product names: Aquascape BioBalls, Atlantic Bio Balls, Matrix-style noodles in net bags.

§03 — Ceramic noodles and rings.

Ceramic noodles and Raschig-style rings are sintered ceramic cylinders sold by the litre or by the kilogram. They have higher density than plastic matrix and are typically used in canister filters where weight is not a handling concern.

Manufacturer documentation cites macroporous ceramic structure as the surface-area mechanism. Sintered ceramic also retains a small amount of water by capillary action when the filter is shut down, which manufacturers position as preserving bacterial colonies through brief outages.

§04 — Sintered glass.

Sintered glass media (Seachem Pond Matrix is the dominant brand) is a porous silicate fired into roughly 10 mm chunks. Manufacturer specifications cite very high surface area per litre — a feature of the open-pore microstructure rather than the bulk shape.

Sintered glass is used in pressurised canister filters and in reactor chambers where compact volume is desirable.

§05 — Moving-bed media.

Moving-bed plastic elements are buoyant injection-moulded plastic shapes (commonly Hel-X, K1, K3) that tumble freely in an aerated chamber. The aeration source is a separate diaphragm air pump and airstone delivering air to the chamber bottom.

The constant motion produces two effects manufacturers cite:

• Self-cleaning — colonies on the outside of each element abrade against neighbouring elements, sloughing excess biofilm before it impedes water flow.
• Maximum oxygenation — the air supply driving the motion is also the air supply for the bacterial colonies' metabolism.

Moving-bed media is used in dedicated moving-bed filter chambers and as an option in some all-in-one box filters.

§06 — Foam as a biological surface.

Foam cartridges are usually marketed as the mechanical stage of a filter, but their open-cell structure also provides a colonisable biological surface. In practice, the rinse interval on a foam stage is short enough that it is best classified as primarily mechanical; manufacturer documentation typically distinguishes mechanical foam from dedicated biological foam grades by cell density.