Many people believe maintaining an aquarium involves a monthly ritual of entirely emptying and cleaning the tank and decorations. But nothing could be further from correct. What most don't realize is that, by doing so, they would be regularly upsetting a desirable biological balance that the fish rely on for their health and well-being...
There are three types of water filtration in an aquarium system: 1) mechanical, 2) chemical, and 3) biological; and they may be used singly, or, more typically, in combination.
Mechanical filtration consists of physically removing solid waste and debris from the system through the use of fiber floss, foam or other similar media to physically trap particles for removal. Even the aquarium gravel itself can be used as a mechanical filter-- more on that later.
Chemical filtration consists of using carbon, resins or other like media to bind waste products at a molecular level. While many believe that charcoal or activated carbon are primarily responsible for filtering aquarium water, they, and other chemical media, are mainly ancillary in nature, providing primarily the function of clarifying cloudy or discolored water-- things that the other two types of filtration cannot do in all cases.
Under biological filtration, naturally occuring bacteria colonize the surfaces and pores of all the other types of media. The gravel on the aquarium bottom can even be utilized as a host for these important bacteria-- again, more on that later. Of the three types, biological filtration is by far the most attractive, because it is both efficient and cost effective.
So you're telling me to grow bacteria in my aquarium? But aren't bacteria harmful pathogens? Won't they make my fish sick? The answer is that not all bacteria cause disease. They are organisms that feed on specific substances and, like all other animals, create waste by-products. In the case of a harmful bacteria-- anthrax, for example-- it is the bacteria's waste products that are toxic to the host organism. Alternately, a bacteria's food source might be the tissues of the organism it attacks. In other cases, however, the actions of bacteria can produce a desirable outcome.
What's unique about the biological filtration in an aquarium system is that fish waste, which is initially ammonia and ultimately the by-products of its bacterial break-down, is the food source of two particular bacteria.
In a process known as nitrification, the first, called nitrosomonas, consumes ammonia and converts it to a substance called nitrite (nitrIte). Nitrite is still very toxic-- nearly as toxic as ammonia-- to aquarium inhabitants. But, luckily, a second bacteria, called nitrobacter, uses nitrite as its food source and converts it to a substance called nitrate (nitrAte). Nitrates are on the order of 20 times less toxic than nitrites are to aquarium animals, given the same amounts.
Therefore, by promoting biological filtration in an aquarium system, chiefly through the use of an "under-gravel filter", the hobbyist is left only to deal with the much-less-toxic nitrates. This is easily done by the monthly exchanging of approximately twenty percent of the aquarium water for fresh, dechlorinated and pH- and temperature-adjusted water.
The under-gravel filter referred to is merely a raised, perforated plate which is placed on the aquarium bottom before a three-inch layer of fine to medium gravel is added. Water from under the plate is pulled to the water's surface through tubes by a water pump or air-driven mechanism. Thereby, the most oxygen-rich aquarium water-- that at the water's surface-- is constantly circulated down through the gravel, the surfaces of which become heavily populated with the nitrifying bacteria. The gravel itself thus becomes a "bio-mechanical" filter in that it physically traps solid waste and debris, allowing the nitrifying bacteria living there to break it down.
Understanding these concepts, it becomes apparent that totally draining and cleaning an aquarium is completely counter-productive to this ongoing biological process. Preserving and maintaining the nitrifying bacteria colonies is of ultimate importance. Clean out the aquarium and gravel-- kill the beneficial bacteria. It would be like setting up a totally new aquarium every month, and most people are aware that the first months of an aquarium's existence are sometimes particularly tenuous, stressful both to aquarium inhabitants and aquarium keepers alike.
Some would still insist that the particles they observe in the aquarium gravel are dirt which must be removed, and a total "tear down" is the only way to get this matter out. In reality, what they are seeing is called "detritus", and, while unsightly, it does not contribute to the decline of water quality. It has been completely broken down by the nitrifying bacteria, and is only a physical remnant of that process. It can be removed without upsetting the biological balance by the use of gravel vacuums or specially designed siphoning devices that remove the detritus without sucking out gravel at the same time.
Armed with the knowledge of basic filtration presented here, anyone has the potential to successfully maintain a healthy aquarium for many years.