Review on Penn-Plax Small World Aquarium Filter & Filter Kit with Replacement Cartridges - Effective Bio Sponge, Carbon, and Zeolite Crystals - Ideal for Freshwater and Saltwater Setups by Becky Evans

This is an absolute must if you use an air pump to filter your small aquarium. The cartridge contains a biocarrier/charcoal and a sponge and these are important ingredients in keeping the 'good bacteria' in your aquarium, in addition to being able to filter and oxygenate the water. Personally, I don't change cartridges very often. What I did do was cut open the bottom of the cartridge to take out the sponge and rinse (make sure you do this in a small container of aquarium water). Then I put it back in the cartridge. This way I can keep the sponge and bacteria clean so the tank can be cycled (and save money). Alternatively, by cutting the bottom you can upgrade with your choice of filler and sponge (see image). I would suggest using an adjustable valve to control the air entering the filter so you can increase or decrease the amount of bubbles. (The filter is a bit noisy, but that's common with any box filter). I've had great experiences and results with this product in my 5.5 gallon betta tank.