bacteria The material has great thickness, it is durable Very suitable for using in freshwater and marine water. Can be cut any sizes as your needed. Lightweight & softness design. Excellent water permeability. Cultivate the nitrifying bacteria for biological filtration. Dense shape makes it effective to avoid residue. Can be used again after cleaning 100% brand new & unused Color: black Advise: Replace the bio-sponge three month, to ensure the filtrating effect as well as the growth of the nitrifying bacteria. Note: There may be an error of 0~2cm in manual cuttingļ¼Those who mind should buy with caution. Package include:1x filter biochemical cotton