JBL CristalProfi greenline External filter for aquariums from 200 – 800 litres. Clean and healthy water Plant and food remains and metabolic substances lead to a deterioration of the quality of the water in the aquarium. Good water quality is necessary for healthy fish and plants. This can be achieved with aquarium filters. The filters suck in the aquarium water and remove the pollution and waste materials from the water. Filters also offer an ideal habitat for bacteria which decompose harmful substances.