I've tried a lot of CO2 diffusers over the years, always searching for the perfect diffuser. Most use a white disc that clogs up quickly to algae. These with the brown diffuser don't grow algae nearly as much. On top of that they are made out of acrylic plastic instead of glass. So they won't break in your hands when you try and install them. We've found this large diffuser to work in any size aquarium and as such only carry the one size due to all the sizes being so close in usability.