Rena nexx is a modular filter designed to grow with your needs for aquariums from 200 litres to 600 litres this external filter capacity and flow grows with each extension addedr the rena nexx allows you to add extensions to the filter increasing its capabilities even morer the filter will give you crystal clear water quality by using the size 4 bio chem zorb cartridger the api crystal is easy and efficient it removes colours and odours and 100 of the water flows through the cartridge