A sponge filter is a great choice for aquariums that need biological filtration, as they are very effective at removing toxic ammonia and nitrite. Establishing the filter correctly is important to ensure it's working optimally and that the aquarium environment remains healthy. The process of establishing a sponge filter can take anywhere from 2-4 weeks depending on the size of your tank and other factors such as water temperature, pH, etc. During this time, beneficial bacteria will begin to colonize in the sponges providing cleaning power for your aquarium. In order to create an optimal environment for these bacteria to thrive, you should make sure you have plenty of oxygen flowing through the system by adding an air pump or additional filters if necessary. Additionally, feeding your fish regularly with high quality food will help promote bacterial growth and keep them in good health during this establishment period. With proper care and attention given to setting up a sponge filter correctly, it can be established within 2-4 weeks so that your fish can live their best lives!