Review on Aquarium Bio Filter Media Sponge: 6pcs Foam Filter Pad Inserts for ML350 - Enhanced Filtration Efficiency by Sean Pere

Perfectly tailored to the size of the rod filter. Shows 3 on each side. Using 2 or 3 will prevent proper water flow, causing water to flow through the overflow channel. I use one in each compartment as a pre-filter. They're enough, of course, to create a biological load, but they still trap most of the food debris. I use different filter threads behind the sponges. This setup works well for good water flow and keeps my aquariums clean.