Hollow ceramic cylinders for emchanical filtration. Traps solid dirt particles at the water inflow and ensures an even flow of water through the filter.
Fluval Pre-Filter consists of inert, solid ceramic rings used to trap large and medium solid particles before they clog the finer sieves of the modules above.
The diversified water flow allows establishment of biological activity on the surface of the rings, adding to the overall biological action on ammonia and nitrite.