I've got about 18KG in mine. I think you need a fair amount if you're going for the Berlin method of filtration to reduce nitrates. You need to make sure you set it up carefully so there's no deadspots (in terms of flow). How many powerheads do you have? Also you will need to get some putty (e.g. milliput) to hold the rockwork together. Take a look at my thread (Electrogear's TL550) to see how I've done my rockwork.