If an aquarium system does not utilize a water pump, compressed air is solely responsible for water movement and gas exchange. Most systems, however, have an underground filter powered by compressed air and an external filter system powered by a water pump. Either way, the air pump is a very critical part of the system. The two types of air pumps generally available to the hobbyist are the vibrator pump and the piston pump. Good brands of each type are reliable and will do a good job.
Maintaining Proper Volume and Pressure
There are two measurements on output from an air pump: volume and pressure. Generally, high volume is achieved with large diameter air system tubes at lower pressure while higher pressure means a lower volume delivered through smaller pipes. In most hobbyist applications, the size of the tubing from the pump to the release is determined by the standard size air tubing, so pressure alone determines the volume of air that moves through a single length of tubing. If a strong pump produces inadequate air from one or two air diffusers (air stones), addition of another air stone may be the best way to increase the air flow to a deep tank, although raising the release in the tank lessens the back pressure and allows a greater air flow.
High pressure is important for pushing air through restricted air diffusers (clogged and/or wood or fine air stones) and for releasing air in deep tanks. High volume is important for providing air to many relatively shallow tanks and/or providing enough air to air hungry applications.
Once the proper air pressure is attained for a particular application, then the volume produced becomes very important. The output in volume and pressure and the size, quality, and price of vibrator pumps is highly variable.
An air diffuser (air stone) receives the output from the air pump and releases the air into the aquarium or a specialized structure. The most common of these diffusers are small cylinders or spheres of a compressed sand-like material, or "air stones", although they may also be wands, blocks, rods, plates, and tubes of various porous materials as well as plastic shipwrecks, clams, and skeletons. High pressure and great volume of compressed air (meaning a good air pump) and an air diffuser with many tiny pores is required to produce many fine bubbles.
Photo by Larysa Johnston
A very good diffuser for tiny bubbles is a wooden air "stone." These are usually made of basswood, silver birch, oak, or limewood - hard woods with very fine, very straight, porous grain - and cut so that the end grain is the largest flat surface. A hole is drilled at the end of the block that extends all the way through the block almost to the other end. A section of rigid air tube fits tightly in place at the opening of the drilled hole. The rigid tube cannotextend very far into the drilled hole or it will block the air release from part of the block. One can purchase a block of basswood and make them up with a fine saw if they can't be found in an aquarium shop.
There are also air stones available made of various sizes of glass beads bonded into small cylinders that are deigned to release bubbles of various sizes. These high quality stones produce a reliable air flow and do not break apart after some use. Air releasers can also be made of natural porous stone, such as pumice, by drilling a hole in a block of the stone as with a wood block. An effective air diffuser that seldom clogs but produces large bubbles can be made by sealing one end of a small section of rigid, plastic tubing and drilling a series of very fine holes around he tubing. A type of ceramic air stone is also available that can be cleaned by heating the stone (minus the plastic tubing, of course) with a propane torch and burning out organic clogs.
Sometimes, depending on the air pump and the water depth, a stone that releases medium or course bubbles is more effective that a fine bubbles release. The quality of inexpensive stones often varies considerable.
Perhaps the greatest air thief is the clogged air diffuser. Always check the air diffuser first when working with an air release problem. Minerals are deposited on surfaces in all aquariums where air and water meet, but especially in marine aquariums. The extremely high mineral concentrations and alkaline pH of saltwater combine to deposit a hard mineral cake wherever there is an air/water interface. These mineral deposits very effectively clog up air diffusers in marine aquariums in a month or so.
Often a clogged high quality air stone can be returned to service. Sometimes just drying out the stone or wood block is enough to open the pores for additional service, but this is usually not very effective. A good stone or wood block can also be soaked in hot water and scrubbed with an old toothbrush to restore air flow. Soaking in alight solution of acid or bleach (1 part liquid bleach to 4 parts water) and scrubbing, rinsing, and then drying them will restore high quality stones and most wood diffusers. Inexpensive stones usually break up under such treatment. Soaking and baking soda solution before scrubbing can also be done with wooden air diffusers. Wood diffusers must be well dried before reinstallation in a skimmer or tank since the wood is swelled when wet and many pores will not reopen until it is dry. (Adapted from "The Marine Aquarium Reference: Systems and Invertebrates")