Filtration is the process that allows us to keep fish alive and healthy outside their natural habitat. It helps clean the water so that the fish can live for extended periods of time without constant aquarium maintenance. Waste products excreted by animals which include ammonia are extremely stressful and life threatening. The primary purpose of filtration is to remove toxic waste products produced by aquarium animals. They work by using a combination of three types of filtration:
Mechanical filtration is the removal of particulates floating in the water column. Generally, this is the first stage of filtration, and is a physical process acting like a sieve as water passes through a medium that selectively traps large particles out of the current. This can help keep the water from being cloudy or silty and make it crystal clear. The medium used can be substrate (such as gravel or rock), sponge, filter cotton floss, a micron sock or diatomaceous earth powder. Mechanical filters will clog over a time period and will need to be cleaned or replaced when this happens.
Chemical filtration generally follows mechanical filtration in most filters. A chemical filter is a medium that chemically traps or releases compounds as the water passes over and through it. There are many chemical filters that can be used to freshen water, but carbon or charcoal is the most common. Carbon is a substance that has many pockets and bonding sites on it, so that impurities, colors, and odors can attach to it, much like a magnet. There is a finite amount of these bonding sites, so eventually the carbon must be replaced. Some chemical mediums (i.e. resins) work like a “sponge” and absorb certain elements and ions from the water. It is important to note that while carbon can help “clean” a tank from byproducts and pollution, it is not a suitable replacement for biological filtration.
This type of filtration is by far the most important for keeping fish in an aquarium. From the moment fish are introduced into the aquarium, they begin releasing ammonia, their principle waste product. If allowed to build, this ammonia can kill fish by damaging the gill tissues and preventing them from taking in oxygen. Beneficial (good) bacteria help us filter the water through biological filtration. Essentially, waste laden water passes over bacteria, which consume the waste and convert it into less toxic compounds.
Nitrosomonas bacteria are traditionally held as the bacteria that are responsible for converting ammonia into compounds called nitrites. The establishment of a healthy reproducing colony of these bacteria takes about 2-3 weeks. These good beneficial bacteria are known as aerobic bacteria (or oxygen loving). This means that this particular type of bacteria needs oxygen in order to live and function. Giving the bacteria a surface area (such as filter media, decorative gravel substrate, etc.) along with oxygenated water will allow for explosive growth of a bacterial population. Once created in the filter media, these bacteria will instantly change all ammonia present in the water into nitrites.
Nitrites, though, are fairly hazardous compounds to fish health as well, so the filter must employ the use of another group of aerobic bacteria to adequately filter the water. Nitrobacter or Nitrospira bacteria are held accountable for the conversion of nitrites to nitrates. These bacteria behave in a similar fashion to the Nitrosomonas bacteria in that they will consume nitrites and excrete nitrates as a waste product. About two weeks are generally necessary for these bacteria to fully seat themselves into a filter’s media. Again, once established, these bacteria will instantly change all nitrites present into nitrates.
The growth and establishment stages of bacterial colonies are what are referred to as “cycling” the aquarium. In all, it takes 4-6 weeks for the cycling to complete. However, once complete, the bacterial colonies will continue to reproduce on their own, sustaining themselves on fish waste until the filter is cleaned.
The initial explosive growth of these bacteria, cause what is referred to as a bacterial bloom. There are so many reproducing in the water column; they will make the water look hazy or cloudy. Do not be alarmed, nothing is wrong as this is a normal process needed to “kick start” the development of these “good” bacteria. Once enough have grown and filled the filter media the rest will die off and the aquarium water will get crystal clear. This usually takes a few days.
To clean a biological filter, it is critical to remember that the media involved house billions of living bacteria, and that the absence of these bacteria can result in the loss of all other living organisms in the aquarium. Usually, the media responsible for containing the bacteria are positioned last in the flow of a filter or are constructed of flow-through materials, so that they collect the least amount of physical waste. When it is necessary to clean it, usage of the aquarium water to rinse the media is essential. Ideally, the rest of the tank is left untouched when cleaning biological media, for it too houses bacteria, and they will help pick up the slack when the main component of bacteria are under stress from cleaning.
TYPES OF FILTERS
There are many filtration options available to hobbyists and newer versions and methods are constantly being tested. Most filters combine all three types of filtration processes, mechanical, chemical, and biological.
INSIDE THE AQUARIUM FILTERS
The biggest advantage of in the aquarium filters is that water stays “in the aquarium”, so there is never a concern of water leaking out of the aquarium. The biggest disadvantage is they, “by design and being inside the aquarium”, are not very powerful so there is a limit on bio-load of fish in the aquarium.
Under Gravel Filters: Under gravel filters are one of the oldest forms of aquarium filtration but still very effective. It consists of a perforated or slotted plate or platform that sits directly on the aquarium bottom with a thick (2-4”) layer of gravel resting on top of it. One or more riser tubes come out of the back section of the plate and rise to the top of the tank. Water is “sucked” through these tubes and “dumped” back into the aquarium. This creates water movement or circulation through the bottom gravel bed, under the filter plate, up through the riser tubes and then back into the aquarium. Two methods of moving this water are used: An air stone at the bottom of the riser tube will blow bubbles that lift water with them on the way up the tube, or a water pump that sits on top of the tube, called a power head, draws water from the tube and expels it into the water column. By passing oxygenated water over a thick gravel bed, we encourage beneficial bacteria to grow all over the gravel. Since a thick layer of gravel has a high surface area, enough bacteria grow as to biologically filter the water. For mechanical filtration, again, the gravel forms an effective stop for most particles in the aquarium. To perform chemical filtration, modules can be placed on top of the riser tube, so that water passes through the media, like carbon. One of the biggest drawbacks of Under gravel filtration is simply due to the design of having large amounts of waste collect underneath the plate. Over a period of time, these large levels of waste will need to be removed from the gravel bed and under the filter plate, not because it is toxic but because it will slow down water movement and take away “attachment” sites for good bacteria.
Sponge Filters: Sponge filters are one of the simplest types of filters available. They have specific uses, and as such may not be suitable across a wide range of aquariums. Their operation consists of drawing water through a sponge with either an air supply or water pump. The sponge performs mechanical filtration, trapping particles in its pores, as well as biological filtration, due to the high porosity of the sponge. The sponge filter has advantages in that the flow is low, reducing risk of damage to weak, small, or injured fish. This is why it is popular for use in quarantine, hospital, and fry-rearing tanks. The cost of the unit itself is quite low, and maintenance is simple to carry out. However, due to the limited size and lack of chemical filtration, its uses are limited to those types of tanks. Additionally, the size of the filter takes up a large amount of area, so it can be difficult to hide.
Internal Power Filters: Inside the tank (internal) filters perform a similar function as undergravel filters only with a different design and mode of operation. These filters draw water through a casing or filter area that contains filtration media, in the form of cotton or synthetic floss, carbon packets, and sponges. These different media perform the mechanical, chemical, and biological filtration and the water is returned back into the aquarium. The one drawback on these filters is that they take up viable fish living space in the aquarium. Internal power filters can however, be a useful alternative format of filtration, especially when the level of water is below the rim of the tank. Most other filters require that the water level be at the top of the tank to operate, but the internal filter is contained completely below the water, so it can be used if the tank is half full.
OUTSIDE THE AQUARIUM FILTERS
These filters have a myriad of advantages over inside the aquarium filters. Because they are located outside the aquarium, the entire inside of the aquarium becomes “livable” space for fish, plants and invertebrates. Again because the filters are not integrated inside the aquarium they can be made large and powerful thus substantially increasing the available bio-load for the aquarium size. The minor drawback is that the aquarium water is removed from the aquarium and sent to an external chamber (filter) so the possibility, even minimal, is that it could leak and water could end up someplace other than it should be.
External Power Filters: These filters are currently the most widely used due to efficient filtration, simplicity of use, and reasonable cost. They hang on the back rim of the tank, with an uptake tube that extends down into the tank. Water is drawn up the uptake tube where it flows into a chamber where media filters the water. Various designs exist, and they usually manage to perform the three types of filtration (biological, mechanical and chemical) efficiently. The main difference found between the many brands and models is how much water is moved through the unit per hour and the ease of media cleaning and changing. For efficient filtration of an aquarium, turning the tank’s volume (in gallons) over 3 to seven times and hour is preferable. This means that for a twenty gallon tank, a filter that pushes 60 to 140 gallons per hour through it is ideal. As the media clogs with detritus, the optimum flow rate may decline, so it generally good to err toward the higher end of flow rates on a filter.
Canister Filters: Canister filters are also external power filters, but generally sit under the aquarium due to their size and weight. They push or pull water through a set of media outside the tank in the filter housing. Canisters are a little different, though, in that they filter the water in a completely sealed, pressurized container. Water is again pulled into the unit through an uptake tube and passes through various media before being expelled to the aquarium again. Canister filters employ powerful pumps to move the water, so the flow rates are often in excess of what might be found in an external hang on power filter. The main benefit of canister filtration is that the filter itself is virtually invisible, since the bulk of it resides beneath the aquarium, in the stand. Additionally, the powerful water movement and exceptionally large space for filtration media allows these filters to be used on large tanks or small tanks with large bio-loads. With quick disconnecting hoses, it is even easy to perform maintenance by taking the filter to the sink, so there is little mess and no spilled water.
Wet/Dry or Trickle Filters: Trickle filters are an evolution of standard filters, trying to be more efficient in less space. The basis on which they were developed is the recognition that aerobic bacteria are the primary focus of all filtration (biological filtration). To maximize populations of bacteria, then, a filter needs to maximize not only surface area, but oxygen as well. Oxygen is much more prevalent in air than it is in the water. Instead of submerging the biological media under water, it is held in a chamber, as water is dripped or sprayed over it. Due to the high contact of air to the wet media, the bacteria grow in a wet-dry, super saturated oxygenated state. This means there is more “free” oxygen left in the water itself for fish bio process since it is not being used to support bacteria life. Bottom line is that more fish biomass can now be kept in the aquarium. Most trickle filters employ a filter fiber as a pre-filter to do most of the mechanical filtration. Bio-balls, DLS material, bio-stars, and more are the media used for biological filtration. Despite the advantage of maximal filtration in minimal size, trickle filters do take up some space, requiring that they typically be located underneath the tank, in the stand, like a canister filter. However, because the system is not sealed and under pressure like a canister, a means of transporting the water to and from the filter is needed. Overflows are the choice of getting water to the filter. These consist of a standpipe in the aquarium or a box suspended inside the tank with a strainer and a box outside the tank connected via a hose to the filter. Water then arrives at the filter, called a sump, where it passes through the pre-filter and into the trickle chamber. After this, the water proceeds to a last chamber containing a water pump to push the water back up a hose out of the sump and into the tank. As previously mentioned, trickle systems can be very useful, especially on large tanks and can be used efficiently on fresh or salt water tanks. They house such incredible numbers of bacteria that once started, will ceaselessly convert all ammonia into nitrate. There are some drawbacks to these systems, however. Size considerations are one factor in deciding if this is the correct system for an aquarium. Many stands will not accommodate appropriately sized filters. The non-sealed aspect of trickle filters can make them prone to overflowing and flooding. It is very important to make sure all the pieces (pumps, overflows, sump size, etc.) work together as a unit. Be sure to perform a “fail – safe” test on the filter system in case of a power failure. When the power shuts off, the tank will drain down to the sump below to a certain level before it stops ---- be sure the sump is large enough to handle this volume of water during the test.
Fluidized Bed Filters: Sand filters, or fluidized bed filters are yet another extension of the attempt to house more quantities of bacteria in less space. The idea is that water is passed through a chamber (from bottom to top) containing sand or sand-like particles. The sand is so small, and has such a high surface area (as compared to gravel, for instance) that high numbers of bacteria colonize them. To work efficiently these filters can be very compact and can hang on the back of a tank or be situated elsewhere. They are sealed systems with a pump on the front end pushing water through the unit. Sand filters can be prone to clogging, as might be expected when large items of debris get pushed through the sand bed, so it is important to have a good prefilter. Despite the high density of bacteria and exceptional filtering abilities, sand filters are not common, as they require much maintenance. They clog frequently, and need replacement of the sand periodically. Because the pre-filter will clog over time, flow rates will vary through the unit. This causes compaction and dead areas of the filter, so in essence the sand filter must be monitored regularly to prevent poor filtration.
Natural Systems: The use of a natural system in an aquarium represents a full circle approach to fish keeping. Instead of trying to filter the water with various engineered filters and media, this method utilizes natural cycles to ensure high water quality. The use of plants in fresh water and live rock and sand in salt water are how this is accomplished.
In freshwater, plants can act as a filter by taking up nitrogen-based compounds like ammonia as a source of food. In aquaria if the quantity of plants is high compared to the number of fish, little else is needed to perform efficient filtration in the tank. Utilization of water pumps to circulate water around the tank to bring food to and take wastes away from plants is necessary for efficient operation. Carbon dioxide injection can also be used to enhance the growth of plants and further the filtration process. A thick layer of gravel (2-4”) may be necessary to help the plants establish roots and grow in health. To properly maintain the plants, adequate lighting must be provided to help them grow. Systems such as these are referred to by the Europeans as aquatic gardens. A special note to be added is that these aquatic gardens are not intended to keep huge populations of fish, but just enough to “accent” the aquatic garden. Due to the large amount of plants, and the night time biology of plants using an enormous amount of oxygen, it would not support a vast amount of fish.
In saltwater, live rock can be used to filter the water. The end results are similar to the natural approach to freshwater, but the mechanisms are completely different. The rock that makes up “live rock” is not actually living, but it provides a mini environment for a multitude of life forms. Bacteria, protozoans, crustaceans, worms, mollusks, echinoderms, corals and sponges all make their homes in and throughout live rock. All of these along with zooplankton and phytoplankton (which are the beginnings of the food chain in the ocean) play important roles in a natural tank in that they add diversity and stability to the aquarium. The bacteria, are the most important, however, as they are the ones doing the bulk of the filtration. These bacteria are similar if not the same as the ones we encourage to grow in our biological filters to consume ammonia and release nitrates as an end product.
This process occurs essentially the same on live rock, but with an important difference. Deep within the rock, there are chambers connected to the outside buy minute fissures and cracks. Water works its way in and out of these cracks very slowly, so that in the interior low oxygen levels abound. Specialized bacteria that thrive in anoxic conditions preside here. These bacteria actually consume nitrates in the same manner as aerobic bacteria consume the ammonia. The end product created by these anaerobic bacteria is nitrogen gas—this bubbles up and out of the aquarium. The result of all of this is that wastes produced by inhabitants of the tank are reduced dramatically by the live rock. This lends incredible stability to the system and allows for high efficiency and long term maintainability, just as it works in the ocean itself. For this approach to work well, it is important to have good circulation around the rock being used. Often the flow is not adequate and the system stagnates, and the bacteria have nothing to neither bring them food nor sweep away wastes. Utilizing a protein skimmer in conjunction with the live rock creates a very efficient system, requiring low maintenance. The skimmer removes proteins from the water before they break down into ammonia, reducing acid production and load on the filter.
Protein Skimmers: Protein skimmers are perhaps one of the most influential developments in keeping saltwater aquariums. They have made it possible to maintain very high levels of water quality for extended periods of time. Protein skimming may also be known as foam fractionation, and the concept behind it has been in use for decades. Protein skimmers function in a deceptively simple manner. Foam is created by mixing dense saltwater (doesn’t work with freshwater) and air together – the finer the mix the smaller the bubbles, the more efficient the skimmer will work. Proteins and other organic molecules stick to the bubbles, which creates a stable foam that rises above the mixing air and water. This foam is then collected before it is returned to the tank.
The wastes, organics, and proteins that are collected are pulled out before they have a chance to break down into ammonia. This effectively scrubs the water clean, removing all manner of waste materials from the aquarium and making the biological filter more efficient. Skimmers do need to be monitored regularly to make sure they are performing well, and not just taking up space and energy. Additionally, because they are so efficient at extracting proteins from the water, they sometimes pull beneficial material like organic molecules that corals use for nutrition and plankton that filter feeders consume. This underscores the need to perform regular water changes, to re-supply the tank with proper nutrients, buffers and salts to make sure all the animals are properly cared for.
UV Sterilizers: Ultraviolet sterilizers work by passing water through a sealed tube with an ultraviolet light; the light emits rays that sterilize or alter the DNA of living organisms that pass by it. Because this is fed by a pump with a pre-filter, large animals (like fish, etc.) are not harmed. Bacteria, protozoan, algae cells and parasites are all killed with this method. The key to using one of these is to size it correctly to the volume of water. UV sterilizers are generally connected “in line” between the aquarium and the filter system. The UV light bulb is effective for about 6 months and then needs to be replaced.