The ZooBest Filterwool blue extra coarse is particularly suitable for the mechanical pre filtering of the water. It holds back rough dirty particles for a long time without clogging. This blue filterwool is also suitable for the biological purification of seawater and for sensitive freshwater fishes exposed to powerful filtration. If it be used as a pre filter medium, the ZooBest Filterwool is to be placed as first layer in an external filter. It can be use in freshwater, seawater and garden ponds. It is free of harmful substances and chemical neutral in the water. Made in Germany.the following box sizes are available:Bag with 100 gBag with 250 gBag with 500 gBag with 1000 gSack with 5000 g