Filter foam gets sold by PPI — pores per inch. Higher PPI means smaller, denser pores. That's the whole rating, and almost every mistake people make with foam comes from misreading what it implies.
What PPI actually controls
Two things move in opposite directions as PPI goes up:
- Filtration fineness goes up. Denser foam traps smaller particles.
- Flow rate goes down. Denser foam is harder to push water through, and it clogs faster.
That trade-off is the entire design problem. There is no "best" PPI — there's only the right PPI for the job you're giving that piece of foam.
The three working ranges
Coarse (roughly 10–20 PPI)
Big pores, high flow, slow to clog. This is your first stage. It catches leaves, uneaten food, and debris before they reach anything finer. Coarse foam is also the best biological media of the three, because water actually keeps moving through it and it doesn't blind over with silt — bacteria need flow, not just surface area.
Medium (roughly 30–35 PPI)
The all-rounder. If you're running a single piece of foam in a sponge filter or a hang-on-back, this is usually the one. Fine enough to visibly polish the water, open enough that you're not rinsing it every week.
Fine (roughly 45–50 PPI)
Water polishing only, and always last in the sequence. Fine foam pulls out the haze that coarse and medium leave behind. Put it first and it will clog in days, drop your flow to nothing, and starve the rest of the filter.
Stack it in order: coarse, then medium, then fine
Water should meet the most open foam first and the densest foam last. Every stage protects the one behind it. Get this backwards and the fine layer does all the work, blinds over, and your flow dies.
In a canister, that means coarse at the inlet side. In a sponge filter, it means the coarse sponge is doing most of your biology and you shouldn't scrub it.
How to clean it without crashing your tank
Rinse foam in tank water, in a bucket, never under the tap. Chlorine in tap water kills the nitrifying bacteria colony you spent weeks growing.
Squeeze it a few times until the worst comes out, then put it back. You are not trying to make it look new. Grey, slightly grubby foam is a mature biological filter. Never replace all your foam at once — stagger it, and let the new piece colonize alongside the old one.
Sizing note
Foam is easy to cut with scissors or a serrated knife. Cut it slightly oversized for a friction fit — a gap around the edge is a bypass channel, and water will always take the easy route around your media rather than through it.