Air pumps are an old workhorse of aquarium equipment. Over the years, aquarium hobbyists have adapted them to a number of purposes, including oxygenating the water and powering aquarium equipment, like pumps and protein skimmers. Air pumps can even power aquarium ornaments. In the modern aquarium, this old standby piece of equipment still finds many purposes.
Many types of aquarium filters, particularly old-fashioned types of filters, are powered by air pumps. This includes sponge, corner/box and undergravel filters. Though old-fashioned, these filters still have a significant role in aquariums. They cost little, and can provide biological, chemical and mechanical filtration. Additionally, while sponge and box filters look somewhat conspicuous in aquariums, their low cost and utility make them ideal for quarantine aquariums. On top of this, with a cheap valve, you can turn down the air flow to the filters and the water flow. This makes sponge and corner filters ideal for breeding tanks, since you can slow down the water flow for the baby fish.
Even without a filter, air pumps can improve certain water parameters. For this purpose, the pump usually is hooked to an airstone, a small, porous "stone" that diffuses the air into tiny bubbles. This increases the amount of oxygen in the water. However, contrary to popular belief, it is not the bubbles themselves that increase oxygenation. Instead, the water movement they create encourages oxygen exchange between the water and air. This promotes oxygen-rich conditions, necessary for certain river-dwelling aquarium fish.
Air pumps also can help protein skimmers function. These devices, popular in saltwater aquariums, function radically different from filters, but still cleanse the water. Protein skimmers create bubbly foam, which lifts protein out of the water column. Some designs of protein skimmers rely on air pumps and high-quality airstones to generate their foam. These designs typically use high-quality wooden airstones, but just about any old air pump can provide the air for protein skimmers.
Since older aquariums often required air pumps, many aquarium ornaments have been designed to run off air pumps. These include standbys, such as the scuba diver with a clam and the self-opening treasure chest. These decorations are powered by air pumps. Typically with these ornaments, air builds up enough in some part of the decoration until it lifts it open and the air escapes, like the lid of a ceramic treasure chest or the top shell of a clam.