quote from Schuran web site : http://www.schuran.com/seawater/nitratfilter_e.html
Suitable for aquaria between 300 and 3000 liters. These data are dependent on the amount and production of nitrate in the aquarium, but on the other hand on the regular feeding. The Nitratefilter is for example able to reduce nitrate values of 20-30mg to a very low value of 5mg in a 3000l tank.