The AquaTop PFE-1 Hang-on Filter is an simple and straightforward (and tiny) hang-on filter that gives you mechanical, chemical and biological filtration. The box itself measures in around 4.2 x 3.2 x 3.8 inches, so you know its not going to be huge, but should work for smaller setups.
The PFE-1 is designed to handle smaller tanks, up to 15 gallons, with a 45 GPH flow rate and a meager 2W of power consumption. Granted, it always seems like these filters are rated for higher volumes, so expect this to be a great option for smaller tanks, closer to 5 gallons than 15.
The AquaTop PFE-1 is built with a filtration chamber system that holds replaceable filter sponge cartridges and the company’s Activated Carbon and Biological Media Trays. They note they are designed to deliver the maximum surface-area for beneficial bacteria, giving them more room to colonize and help keep water clean.