Fluval FX6. I have an FX5 (older version) and it works fine on my 640 litre. I don't have any fine media in my FX5 so it needs cleaning only every few months and then mostly for the pipes that get a bit 'algaefied'. To take the fine particles out I use a Fluval U4 internal that has only fine filter material in it as it is very quick and easy to clean, takes about five minutes during a water change.