Air pump and filter?

The relationship between air pumps and filters in aquariums is often misunderstood, yet fundamental to establishing a thriving aquatic environment. In essence, a mechanical filter (such as your Marineland Penguin Pro 125) serves three purposes: mechanical debris removal, chemical filtration, and most critically, biological filtration. This filter continuously draws water through various media, fostering colonies of beneficial bacteria that metabolize harmful ammonia and nitrite. Such filtration is absolutely essential for maintaining water quality and supporting aquatic life.

The air pump, on the other hand, serves a different, more subtle, role. Its principal contribution is not filtration, but aeration—promoting gas exchange at the water’s surface, enhancing dissolved oxygen levels throughout the tank. While most modern power filters already agitate the surface sufficiently to allow for adequate oxygenation, the air pump can provide added insurance, especially in densely stocked tanks, warm tanks (where oxygen saturation is naturally lower), or during power outages if run on a backup.

For your 20-gallon setup, the combination isn't strictly necessary, but can be symbiotic: the filter for water quality, the air pump for added oxygen and visual effect (such as bubble walls or ornaments). Retain both but know their unique functions—one preserves life, the other enhances it.