Ozone has many uses in the aquarium but it is mainly used to kill disease causing organisms. Because it kills on contact it is strongly recommanded that it is used in conjunction with a redox controller and that it is used in a sump type set-up. There are 5 models in the range, the 25 is for aquariums up to 250l, the 50 for aquariums up to 500l, the 100 for aquariums up to 1000l, the 200 for aquariums up to 2000l and the 300 for aquariums up to 3000l.