Aqua One Premium Sub is an extremely porous sintered glass biological media that can be used in all types of filtration systems! Pressurised canister filters, hang on filters, internal filters and wet/dry filters just to name a few. Premium Sub provides the most ideal environment for the cultivation of denitrifying bacteria, due to its porous nature. Premium Sub is a key component to a healthy and productive biological filtration system. Suitable for both freshwater and saltwater aquariums.