30ppi Hamburg Mattenfilter for 10, 15, 20L and 33L gallon tanks

All About Matten Filters Including How To Make One

Mattenfilter Foam — Volker's Aquarium Supplies

The HMF – What's That? — Seriously Fish

Building a Hamburger Matten Filter (HMF) for an Aquarium : 4 Steps - Instructables

Mattenfilter Foam — Volker's Aquarium Supplies

Blue Pet Aquarium Internal Liquid Filter BL-601F Suitable for 20-30 Litre Aquarium Fish Tank (Power: 6W, Flow MAX: 600 L/H) : : Pet Supplies

DIY Matten filter for aquariums

Mattenfilter - SWISSTROPICALS