Review on Lefunpets Sponge Filter: Perfect Breeding Fry Betta Shrimp Nano Fish Tank for 5 Gal-80 Gal Aquarium Fish Tank by Amanda Edwardz

Unless you plan to modify the filter with an airstone, I recommend using the filter without a sponge to ensure it is working properly. Make sure air is ACTUALLY pushed into the bottom of the filter to create a vacuum. All the air came out at the junction of the down pipe and the top of the filter. I worry that many people use faulty equipment (or this product just doesn't work without modification) without realizing it because it still creates bubbles. Medium if you want to run this thing without an air stone.