Aquarium and pond filter foam.

Foam earns its place in an aquarium filter twice over. Mechanically, it strains particles from the water through its depth rather than across a face, so it keeps flowing long after a pad would have blinded. Biologically, the enormous surface area of the strand lattice is colonised by the nitrifying bacteria that convert ammonia to nitrite and nitrite to nitrate.

That second function is the one people underrate, and it changes how the filter should be treated. The foam is not a disposable pad — it is a living part of the system, and a mature block is worth considerably more than a new one.

The single most common mistake in a sump or canister is putting fine foam where the water enters. It clogs within days, and once clogged the flow finds a way around it rather than through — so the filter is both restricted and bypassed.

• First chamber, 20 – 30 PPI. Takes leaves, debris and larger waste without loading up.
• Second chamber, 45 – 60 PPI. Polishes the water after the coarse stage has done the heavy work.
• Biological volume. Any grade hosts bacteria, but the coarse stages keep flowing, and flow is what feeds the colony.

Ponds follow the same logic at larger scale, with an even coarser first stage because the debris load is seasonal and heavy. See choosing a PPI.

This matters more than anything else on this page. Rinse aquarium foam in water taken from the tank, not in tap water. Chlorinated tap water kills the bacterial colony you spent weeks establishing — and a filter whose biological capacity has been wiped out can trigger an ammonia spike that harms fish days later, long after anyone connects the two events.

• Rinse in tank water. Squeeze gently in a bucket of water drawn from the aquarium.
• Accept that it stays discoloured. Clean-looking foam has usually been cleaned too well.
• Stagger the chambers. Never clean every piece of foam at once — do one chamber at a time so the colony survives.
• Replace by feel, not schedule. When the lattice has lost its spring and starts crumbling, it is done. Replace one block at a time.

Foam is cut to your chamber — blocks for a sump, discs and rings for a canister, sheets for a box or trickle filter, and slots for internal baffles and pipework. A light interference fit is usually best, so water passes through the foam rather than channelling around its edges, which is the aquatic equivalent of a leaking filter frame.

Send the chamber dimensions or the old foam. See die-cutting to your drawing.