Review on Optimized Aihotim Submersible Foam Filter, Ultra Mini Aquarium Single Sponge Filter, Whisper Quiet Fish Tank Filter, for Small Fish Tank 0.5-5 Gallon, Ideal for Freshwater and Saltwater Environments by Jill Bethel

I got it for my 5 liter tank. I had it for about a month. In general I am satisfied. It keeps the water fresh and I don't worry about my betta's fins. However, the suction cup does NOT stick. The item is heavy enough to still stay underwater and I've completely given up trying to get it to stay on the side of the glass. While it works well, it's probably better for a small tank. Please also note that you MUST purchase an air pump separately for this.