What will you place after the skimmer compartment ? The compartment where the skimmer is will have bio media, basically whatever i have accumulated from my current tank will be moved over. (marinepure block, marinepure block, siporax, bacteria king) The 3rd compartment will be a refugium. The underflow area is covered with a netting to prevent fishes from getting into the return pump section. As the name implies, it'll be a refuge area for injured fishes (if any) My suggestion instead of using net, some fish can be trapped by net, you might want to consider sandwiching the underflow glass with 2 pieces of egg crate and glue it secure. Will block fishes passing through the underflow portion and yet do not trap fish and block water flow. Just a thought. Sent from my iPhone using Tapatalk