Sponge Filter

This is the most basic of all filters and requires at least three components: a sponge filter (located inside the tank), an air pump (located outside the tank), and airline tubing to connect them. An air pump pushes air through a tube into a cavity inside the sponge filter. The rising air bubbles draw water through the sponge walls, mechanically collecting debris from the water and allowing beneficial bacteria to grow.
Pros: I could keep using it, but the device is cheap, easy to clean, and hard to break because it has few mechanical parts. It provides good water circulation and surface agitation while being gentle enough to avoid sucking up fry, shrimp and other slow-moving creatures. Plus, during a power outage, the beneficial bacteria on the sponge will remain in the oxygenated tank (giving it a longer chance of survival), and you can even buy a battery-operated air pump to be prepared for emergencies.
Disadvantages: Sponge filters take up physical space in your fish tank, so you may want to hide them behind rocks, plants, or other aquarium decorations. Also, there is no way to add chemical filtration if needed. I personally don't like the bubbling sound a sponge filter makes, but that's easily fixed with a little air stone.
Bottom line: Sponge filters are often found in fish stores, fish halls, and breeding facilities because they are so reliable and cost-effective. Why not use something tried and true?