Pond Filter Media — K1, Bio Balls, Sponge and More

Filter media is what does the actual filtering inside your pond filter. The filter housing is just a box; the media is what catches debris and grows the bacteria that keep your pond water safe.

Mechanical media

Catches physical debris before water reaches the biological stage:

Coarse foam pads (10–20 ppi): first stage, traps leaves, twigs, fish waste. Replace every 1–2 years.
Fine foam pads (30–45 ppi): polishing stage, traps fine particles for crystal-clear water.
Japanese filter mat: high-end mechanical media that lasts 5+ years. Cuts to size with scissors.
Filter floss: disposable cotton-like media. Cheap, very fine, replaced monthly.

Biological media

Hosts the bacteria that convert ammonia → nitrite → nitrate. Surface area is the key spec:

K1 / Kaldnes (moving bed media): the gold standard. The pieces tumble in aerated water, growing massive bacterial colonies. ~750 m²/m³ surface area.
K1 Micro: finer K1 for smaller filter chambers and aquariums.
Bio balls: static media. Cheaper than K1 but lower effective surface area — biofilm only grows on the outside of static media.
Ceramic rings: porous ceramic media, very high surface area inside the pores. Great for canister filters.
Lava rock: budget option, high surface area, heavy.

How much media do you need?

For biological media, a rough rule: 1 litre of K1 per 1,000L of pond water for goldfish, double that for koi. Mechanical media depends on the filter housing — fill all the chambers it has.

Cycling new media

Brand-new media has no bacteria. The filter will run ""uncycled"" for 4–6 weeks until the colonies establish. Speed it up with a bacteria starter. Test water during cycling — ammonia spikes can kill fish in a single day.