Colonize beneficial bacteria in your aquarium and block debris with Fluval C4 Bio-Screen Pad Filter Media. One of the most important ways to maintain good water quality and keep your fish thriving is seeding your aquarium with good bacteria. This bacteria breaks down harmful ammonia from fish waste and other organic matter and starts the aquarium nitrogen cycle. Reducing debris can prevent odor and harmful ammonia levels. Fluval C4 Bio-Screen Pad Filter Media provides a large biological surface area for beneficial bacteria to grow and it blocks debris. Plus it improves the biological filtration process by evenly distributing the water flow before it drips into the biological trickle chamber. The results is clean, crystal clear water.