Decomposers are living organisms, that get any extra food from other dead organisms. They keep all the nutrients from the dead organisms so it's available to other plants and animals. Any of the left over nutrients, they use to create bodies for any other animals to eat. Other nutrients gets passed through the decomposers, turns into waste and it goes into the soil and dirt for other plants and animals to take in.
Water filters aren't the only thing that cleans water. The forests and wetlands clean water too. The flow in the water is slowed down, and the vegetation traps some of the sediments and pollutants.
Other than plants, aquatic animals clean the water too. Example: Fresh water clams clean water by pumping water through their bodies to filter out all the water and food. It's also cleans the clam as well. And that's how they live.