Biological filtration refers to the usage of beneficial bacteria and aquarium plants to consume the toxic nitrogen chemicals generated from fish waste, thus purifying the water. Because beneficial bacteria grows on any surface area in the tank that is well-oxygenated, the filter is a primary location to boost the population. Biological media (such as bio rings and bio balls) come with lots of porous or intricately patterned surfaces to serve as “housing” for the bacteria colony. (In fact, beneficial bacteria also readily grows on the coarse sponge pads used for mechanical filtration.) The shape of the bio media also allows water to flow freely though them to bring more oxygen to the bacteria. Aquarium gunk can cover these surfaces over time, so clean the bio media every 1-3 months by gently waving or rinsing it in old tank water until the debris falls off.