Aquarium filters probably generate more questions, and are fraught with more mysteries than perhaps any other topic in aquarium keeping. Much of this mystery is deliberately generated by filter manufacturers, all of whom want you to buy their product. Few filter manufacturers are innocent here. I have seen so many preposterous claims coming from filter manufacturers, it borders on deceptive advertising. I believe this is a gross disservice to the fishkeeper, who may or may not be able to read advertising hype, and sort through that which is important, and which is just whistles and bells and decoration and the product of the fertile imagination of an advertising writer.

  Let us examine the basics for a minute. In nature, water is replaced constantly (as in a moving stream), or is held in large quantities (as in a lake), so that any waste or debris produced by fish is quickly washed away or diluted. In a home aquarium, this does not happen. Waste materials are excreted into the aquarium's relatively limited water supply, where it builds up. Plants, too, do their part to contribute to the buildup of debris. As plants grow, small bits break off and enter the water column. If not removed, they will eventually decay and break down, dumping their own waste byproducts into the water. Remember that we also add fish food to our tanks, every day. This fish food never leaves the aquarium; it simply gets converted to fish poo, or it settles into the gravel where decay processes work on it, converting it into ammonia and nitrates.

  Some of this waste material can be removed by growing plants. Plants can consume ammonia directly, and nitrates also serve as fertilizer to plants. However, unless the fish load is very light and the plant load is very heavy, it is likely that waste byproducts must be dealt with in a direct manner and be forcefully removed from the tank. Additionally, there can be some products of decomposition that cannot be removed by plants, and must be removed by water changes.

  Filters come in many varieties. Glancing through an aquarium supply catalog shows we have choices of many different types of power filters, canister filters of many sizes, shapes and descriptions, and other filtration devices. Which one is the correct choice for our tank? Is one better than another? Are there any to be avoided?

  To answer these questions, we need to know what we are trying to accomplish. Generally, filtration falls into one of three categories:

Mechanical Filtration:

  This is the physical removal of mechanical debris in a tank. Bits of solid waste, bits of plants, miscellaneous tank litter, all are removed via mechanical filtration. A mechanical filter is simply some sort of screen or fine sieve material that catches this debris and holds it until it can be removed from the tank and discarded. We can accomplish this by passing water through any fine, porous media. One of the earliest mechanical filter media was filter floss. Originally made from either cotton or "glass wool," i.e., finely spun glass filaments formed into a mat material, this method worked well at trapping debris. Later, polyester was utilized by forming it into a floss batting material. Polyester filter floss has advantages over cotton or glass wool. Cotton, being an organic material, will itself begin to decay and to dump organic waste byproducts into the water. Glass wool was not the safest stuff to work with, as the glass fibers could get into your skin. If you happened to get it into your eyes, it has a very irritating effect that could actually last for years. Polyester floss has none of these drawbacks. It releases nothing into the water, and catches microscopic debris very efficiently. It's relatively cheap when bought in bulk, and can be discarded and replaced when dirty for only pennies in cost.

  Polyester can also be formed into a porous mat, where it may be attached to a plastic frame and inserted into guide rails so that water can be passed through the mat. This process is used in many power filters today. Examples of filters that use these formed mats of polyester would include Penguin, Millennium, AquaTech, AquaMaster, Emperor, etc. It is an efficient way to trap mechanical debris and remove it from the water. However, when sold in filter cartridge form, it is frequently sold with activated carbon sandwiched inside, and the aquarist has no other choice but to run carbon in his tank. This may or may not be desired.

  Another efficient mechanical filter is an open-pore sponge. Water passed through the weaving, convoluted channels of a porous sponge is cleaned of mechanical debris very efficiently. An advantage of a sponge is that it can be rinsed and re-used indefinitely. So can polyester filter floss formed into cartridges, but the manufacturers don't seem to want you to know this. An example of a manufacturer that uses sponges as a mechanical media would be AquaClear.

  There are other mechanical filter methods. Most canister filters use small ceramic rings, which catch larger debris and slam it to and fro inside the filter so that it is broken down into smaller and smaller debris.

  One canister filter manufacturer uses a plastic mesh that looks sort of like a large plastic scouring pad. This traps smaller mechanical particles. One great advantage of this media is that it can be rinsed and re-used indefinitely. An example of a manufacturer that uses this plastic mesh is Eheim.

  In recent years, a new filter media has gained in popularity. Although intended primarily for biological filtration, sintered glass materials also contribute to mechanical filtration, removing very fine particles from the water column. Filters using sintered glass materials such as Siporax, BioMax, Ehfisubstrat or Seachem's Matrix tend to produce extremely clean and clear water.

  Any of these media may be used in various combinations in filters. The most efficient method of using these media is to arrange them so that the most coarse material filters the water first, to remove large chunks of debris such as plant leaves or large pieces of fish poo. Next should come filter media of medium consistency, to remove even finer debris. Finally, the water can be "polished" by using the finest filter media, to trap fine and even microscopic particles. Used in this order, filters perform their filtration at a very high rate of efficiency.

BIOLOGICAL FILTRATION:

  While mechanical filtration is, by far the most difficult task for a filter to perform, it is equally important to remove dissolved organic waste material. Fish excrete waste in the form of ammonia. If allowed to build up, ammonia would quickly rise to toxic levels, killing the fish. Imagine yourself being forced to live in a septic tank, and I think you get the idea.

  This ammonia, however, is converted by beneficial bacteria to less toxic forms. Ammonia is used as food by bacteria commonly referred to as Nitrosomonas species, and converted to nitrites. Nitrites are also extremely toxic, and would also kill fish if allowed to build up. Fortunately for us (and our fish), other bacteria commonly referred to as Nitrobacter species use nitrites for food, and convert them to forms of nitrates. Nitrates are much less toxic to fish, and must build to high levels before becoming a problem. Some fish are more tolerant of nitrates than others. Goldfish and Koi seem to be able to tolerate nitrate levels much better than tropical fish. However, we don't want to let nitrates build to too high a level. We will discuss nitrate removal in more detail in a few minutes. Suffice it to say for now that no commonly available commercial filters can remove nitrates.

  As mentioned, biological filtration takes place via beneficial bacteria. These bacteria live in the slime coating that develops on all smooth surfaces of the tank. They live on the glass walls of the tank, on the rocks, on the gravel, on the decorations and ornaments, on the heater, on the thermometer, on Mickey as he opens the treasure chest. Some of these bacteria can also be found inside the filter itself. Remember, bacteria will grow on any smooth surface, and that would include the inside walls of the filter, on the impeller blades, on the inside of the filter intake tube, and on that little paper wheel spinning in the current. Filter manufacturers deviate far from the truth, though, when they claim that ALL the biological filtration takes place ONLY in their filter, or on that paper wheel. Bacteria are not cattle; they cannot be herded into one area only. They will grow throughout the tank, wherever they can find a surface upon which to attach. The filter box and its contents provide less than 15% of the total surface area available to bacteria. Therefore, it is readily apparent - and measurable - that less than 15% of a tank's biological filtration takes place in the filter or its media. MOST biological filtration takes place in the tank, not the filter.

  Some filter manufacturers can force a larger percentage of bacteria to grow in the filter, however, By providing relatively large surface areas for bacteria to grow on, they simply provide more places for bacteria to spread out and grow. In the past few years, sintered glass products have come on the market that do seem to perform a larger portion of the biological filtration, if only because of their tremendous surface areas provided. Siporax, Ehfisubstrat, Seachem Matrix, Fluval's BioMax, Cell Pore, all are examples of this technology. I have seen different claims by different manufacturers. Some say that one ring of their product will provide the biological surface area of a tennis court. Another talks about how using a box of their product will provide literally acres of surface area. I have no way to prove or disprove their claims, but in theory I believe their claims are probably not overly exaggerated. Whatever the actual surface area provided, it is certainly going to be much larger than provided by the tank's inner surfaces. In filters loaded with sintered glass or glass/ceramic foam products, it is likely that a large portion of the biological filtration load will occur in their media. Of course, it will never provide 100% of the biological filtration, simply because bacteria spread out evenly over their available surface area. Some bacteria will always be present in your tank, its gravel, its rocks, etc.

  It is important to remember that bacteria and their food (ammonia, nitrites) are always in balance. In an aged tank, the quantity of bacteria will reach levels adequate to process out ammonia and nitrites, and will seldom rise or fall in total numbers to handle this load. Thus, in a lightly loaded aquarium with only a few small fish, the overall numbers of bacteria will also be low. In a heavily loaded aquarium with lots of fish, the overall numbers of bacteria will be high. They will always seek equilibrium: just enough bacteria to consume just enough food.

  Thus, huge surface areas for bacteria to populate will usually be wasted. Most of the surface areas will be lightly loaded as the bacteria spread throughout their available surface areas. Another way to say this is that most filters that provide huge surface areas for bacteria end up wasting most of that surface area. However, it can be comforting to know that the extra surface areas are there in case they are needed. If, for example, you should happen to accidentally overfeed your fish (hopefully it's not something you do all the time), the ammonia generated as the food decays can be handled by the tank's beneficial bacteria. They will simply multiply in numbers (over several days) to process the available waste, and then their numbers will drop back down as the food is consumed.

  It quickly becomes apparent that the actual number of bacteria in a tank rises and falls as the waste load of the tank rises and falls. Bacteria are constantly being "born," live their short lives consuming ammonia and nitrites, and then they die, to be replaced by new bacteria. The bacteria living in your tank today are not the bacteria that were in your tank last week. Today's bacteria are the offspring of yesterday's bacteria.

  Understanding this, it now becomes possible to visualize what is happening in a filter. Some of the tank's bacteria live in the tank, and some live in the filter. The filter is by no means providing the sole environment for biological filtration. This is why you can change out a filter cartridge without stopping the biological filtration process. The bacteria on the cartridge are relatively few in numbers, and are quickly replaced as other bacteria multiply to pick up the slack of bacteria lost on the old filter. This is also why a tank will not "crash and burn" if the filter should happen to fail. I see frequent advice that one should run several filters on a tank, in case one should fail. Well, if you don't expect to be able to find a replacement for 3 or 4 weeks, then this theory might be useful. Also, if you practice poor fishkeeping habits and chronically overcrowd your tank with way too many fish, then perhaps an extra filter on the tank might help. The main purpose of running several filters on a tank, however, is simply to extend the time period between filter cleaning. This is a foolish practice, however, because by running several filters on a tank and extending time between servicing the filter, you are only leaving the trapped debris to slowly decay in the water's path. Thus, running extra filters may actually be defeating the purpose of running a filter (by leaving waste material in the water stream longer), and may actually be creating MORE waste byproducts to build up in the tank, not less. By running one filter and servicing it more frequently, your water will probably be more clean than by running several filters and servicing them less frequently.

  In choosing a filter, therefore, one should choose ONE adequate quality (and therefore reliable) filter and plan to maintain it regularly, rather than to run multiple filters with the goal of longer intervals between maintenance.

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