An aquarium airpump was once a necessary piece of equipment for all fresh and marine aquariums. Airpumps are often diaphragm-driven and force air through a tube, into an airstone, and into the aquarium. Airpumps can be used to increase dissolved oxygen within the aquarium, and the air flow upwards creates a small volume of flow. This technique of increasing flow has been used to draw water through filter material (corner or box filters) and substrate (under gravel filters). The pumps can be purchased with a plug for standard electricity outlets or battery operated. The battery operated air pumps can be invaluable during power outages, retaining some oxygenation and water movement when all other functions have stopped. Airpumps can be a wise investment considering their relative cost, and battery powered pumps can be an appropriate edition to emergency equipment.
Airstones (that sit on the end of airtubing) can be ceramic, plastic or wooden. The wooden airstones often have the smallest bubbles (and hence better oxygenation), but they do not last as long when submerged constantly as the wood rots and expands. The classic "blue" airstones are made from coloured sand, and have moderate bubbles, but do break if dropped. Plastic airstones can last longer than others, but are hard to adjust and have poor bubble size.