Aquarium & Pond Filter Foam: How to Choose PPI, Density and Shape
Aquarium & Pond Filter Foam: PPI, Density and Shape
GaoLeshi Technical Note | Reticulated Filter Foam | Updated 2026
Two jobs, two different foams
Water filtration asks foam to do two contrasting jobs at once. Mechanical filtration strains out suspended solids — uneaten food, plant matter, fish waste. Biological filtration provides surface area where nitrifying bacteria establish and convert ammonia. The foam that excels at one is rarely ideal for the other.
A fine-pore foam polishes water beautifully but clogs within days, and once clogged it cannot host bacteria effectively. A coarse foam runs for weeks without cleaning but lets fine particles through. Understanding this trade-off is the whole art of selecting aquarium and pond foam.
The recommended three-stage stack
| Stage | Position | Suggested PPI | Job | Cleaning frequency |
|---|---|---|---|---|
| Stage 1 | First contact with dirty water | 10–20 PPI | Catch coarse debris, protect downstream | Every 1–2 weeks |
| Stage 2 | Middle | 20–30 PPI | Remove medium suspended solids | Every 2–4 weeks |
| Stage 3 | Final / bio stage | 30–45 PPI | Fine polish plus bacteria habitat | Rinse gently, monthly |
Density: the parameter most buyers skip
In water duty, density governs two things that matter daily. First, structural durability — low-density foam tears during repeated cleaning, and a torn filter is a bypass. Second, compression recovery — foam that stays compressed loses both flow and surface area.
| Density | In aquarium / pond service |
|---|---|
| 18–25 kg/m³ | Lightweight and cheap; suitable for disposable or very lightly loaded systems, but tears more easily during cleaning |
| 25–35 kg/m³ | The practical default — survives repeated rinsing while staying open |
| 35–50 kg/m³ | For high-flow pond systems and long service intervals; heaviest and most durable |
Choosing a shape
Sheet & blockMost common
- Easiest to size to an existing compartment
- Any thickness, die-cut to shape if needed
Tube & cylinderInline filters
- Full-depth flow through the wall
- Custom inner and outer diameter, length
Custom die-cutOEM equipment
- Repeatable tolerances for production runs
- Send a drawing or sample for quotation
Carbon-loadedOdour & discolouration
- Handles both particulate and dissolved organics
- Carbon loading adjustable to requirement
Care and replacement
Rinse in removed tank water, not tap water
Chlorinated tap water kills the bacteria colony you have spent weeks establishing. Squeeze the foam in water taken from the tank during a water change instead.
Never clean all stages on the same day
Cleaning every stage at once resets the biological filter and can trigger an ammonia spike. Stagger the stages a week apart.
Replace when structure fails, not when it looks dirty
Discolouration is normal and harmless. Retire the foam when it no longer springs back, tears easily, or has permanently lost its shape.
Frequently asked questions
What PPI should I use for a pond filter?
Start at 10–20 PPI for the mechanical stage where debris load is heaviest, and finish around 30–45 PPI. Pond systems carry far more solids than aquariums, so the coarse stage does most of the work.
Can the same foam serve as both mechanical and biological media?
Yes, and that is what most hobbyist filters do. The practical approach is to let the upstream coarse layer handle solids and accept that the finer downstream layer will host most of the bacteria.
Is the foam safe for aquarium use?
Our filter grades are open-cell polyurethane without added dyes or plasticisers in the standard black, grey and white versions. For a specific compliance requirement, tell us the standard and we will confirm the grade and supply test documentation.
Do you supply tubes and cylinders for canister filters?
Yes. We produce hollow cylinders and tubes in custom inner diameter, outer diameter, length and PPI, including carbon-loaded versions. Samples are available before bulk orders.