What's filter media for?
A simple question with a simple answer: filter media is used to clean the water in the aquarium. The filter, whether it is air powered or electrical, uses a pump to draw water through the filter media, and in doing so solid wastes (such as fish droppings and dead leaves) are dissolved wastes (primarily ammonium, the fishy equivalent of urine) are neutralised.
There's obviously two things going on here, and to some degree they are mutually exclusible. In other words, the more solid waste the filter removes from the water, the more clogged up the filter becomes. The more clogged up the filter is, the slower the water moves through the filter media, and the slower the rate at which ammonium is neutralised. This is why regularly cleaning a filter is important.
Different types of filter media
Filter media can be divided into 3 basic types: mechanical filter media for removing solid wastes; chemical filter media for chemically removing dissolved wastes; and biological filter media, which support the bacteria that neutralise dissolved wastes. Some types of media can perform multiple tasks, while others are specialised for just one thing.
The most basic filter medium is sponge. Most internal and external filters use some type of sponge, and many will have two or more different "grades", a coarse sponge and a fine sponge. The coarse one will be the mechanical filter, while the finer one will be the biological one. Sponges work well because they can be easily cleaned and reused (making the ideal mechanical filters) but have enough of a surface area to support lots of bacteria (allowing biological filtration).
The instruction manual for your filter will show you how to arrange the sponges if you have more than one type. Normally, the coarse filter is placed where the water comes into the filter, and fine filter before the water is pumped out and back to the tank. Regardless, the basic rules are the same as those for any filter medium. You can clean mechanical filters in hot water if you want, but filters used as biological filters must only ever be cleaned in a bucket of aquarium water. Hot water will kill the bacteria, wiping out the biological filtration capacity of the filter medium.
Sponges can be cleaned and reused almost idefinitely.
Ceramic chips and tubes
There are all sorts of ceramic chips and tubes sold, such as Siporax, Bio-Max, and EhfiSubstrat. Like sponges, these are often sold in mechanical (or "pre-filter") and biological grades. Again, it is important to clean biological filtration media in aquarium water not hot water. In theory at least, the mechanical grade filter media doesn't have the same surface area per unit volume as the biological grade media, and so isn't the best choice for biological filtration. This is reflected in the price difference, the biological grade media being significantly more expensive.
Manufacturers will suggest ceramic media is changed every few months. You are free to do this if you wish, but remember not to change all the biological media at once. Changing half the biological media at a time will allow bacteria from the old media to colonise the new stuff, maintaining the cleanliness of your aquarium. Personally, I prefer to clean and reuse ceramic media, and have found the good quality stuff at least to last (and work well) for many years.
Filter wool can be used in many ways. Some filters use a small amount as a pre-filter (this is the case with the Compact-H filters in the Juwel aquaria, for example). Others use filter wool to "polish" the water, placing it at the end of the canister filter, just before the pump, and so ensuring the water sent back to aquarium is spotlessly clean. Filter wool can also be used as a the mechanical or biological media in a canister filter. Loosely packed wool works well as a mechanical filter, while tightly packed wool has enough of a surface area to support a decent colony of bacteria.
While versatile, the downside to filter wool is it gets dirty very quickly and is difficult to clean. In the average community tank, wool will need cleaning within a month or two, and it can only be cleaned a few times before it completely falls apart. I find I need to replace the stuff completely every 6 months or so.
Activated Charcoal (Carbon)
Activated carbon is a chemical medium. It has a highly fractured surface, and any organic compounds that come in contact with the surface of the carbon stay there. This process is called "adsorption" (not absorption, which is something else entirely). Because of the fracturerd surface, each carbon grain has a large surface area, and will adsorb a lot of stuff, but eventually the carbon becomes saturated, and needs to be replaced. In theory, it can be cleaned and reused by being baked in a hot oven, but there's not much point since carbon is usually very inexpensive.
The question for many aquarists is why use carbon. Back in years gone by, before filters were as effective as they are now, aquarists tended to leave weeks, even months, before water changes. The idea was that the stability of the aquarium was the thing, and water changes disrupted the equilibrium. Because of the way these tanks were being run, the water became "old", and acquired a yellowy tint from the organic chemicals that were building up in the aquarium. Carbon adsorbed these chemicals, known by a German word, "gelbstoff", and so got rid of the yellow tint.
In a modern, well-maintained aquarium, there should be no build up of organic chemicals in the water. Hence, carbon isn't strictly necessary. Moreover, carbon has an important negative attribute: it adsorbs fish medicines. If you are treating for something like whitespot, it is vital that any carbon in the filter is removed. Otherwise, the carbon will neutralise the medicine, and stop your fish getting better.
It is also important to realise carbon is an excellent medium for filter bacteria. After a few weeks, the surface of the carbon is so completely covered in bacteria that any adsorbtion properties it orginally had will have diminished almost to zero. Hence, if you want to use carbon as a chemical medium, you need to clean it and/or replace it on a regular basis (i.e., every few weeks).
Zeolite (Ammonia Remover)
Zeolite is a mineral that binds with ammonium, and in doing so removes that ammonium from the water. This is obviously a good thing in an aquarium, and zeolite is often marketed as a "wonder media", a cure-all for immature aquaria.
It isn't quite that simple. Ammonium removed chemically can't be used to feed filter bacteria, so using zeolite in a young aquarium will only delay the development of the biological filter. Moreover, once the zeolite is saturated, it won't accept any more ammonium, and without a viable biological filter, that ammonium is going to build up and poison your fish. Finally, zeolite will not work in brackish or marine aquaria, because while zeolite takes ammonium in freshwater conditions, it surrenders that ammonium in salty water.
Personally, I don't have any use for zeolite except in emergency tanks such as ones set up to hold an unexpected batch of baby fish. A healthy biological filter is far more reliable alternative for most other aquaria.
Coral sand and peat
Coral sand isn't a very effective mechanical filter medium, and only a so-so biological one, but it does have the advantage of buffering aquarium water, raising the pH and hardness. In tanks with fish that need hard, alkaline water, such as brackish water fish and livebearers, adding some coral sand to the filter can be worthwhile. In marine tanks, such an approach is barely adequate, and you need to find a more effective way to stabilise the water chemistry, something that falls outside the scope of this essay.
To create the reverse situation, water with a low pH and hardness, peat is often recommended. Peat granulate is probably the optimal choice here since it is concentrated and can be used in small amounts. Peat doesn't serve any value as a mechanical medium (quite the reverse, as it tends to fall apart), and it isn't a very good biological medium either. Use peat as a supplement to your primary filter media rather than an alternative. You can buy small nylon pouches into which a little peat granulate can be added. Pop the pouch into the canister filter, and away you go. Be sure and check the pH of the aquarium on a regular basis; in soft water especially, the pH drop can be very rapid, and even soft water fish like angels and neons won't appreciate that.
How do you choose?
Think about what you want your filter to do. The prime functions are mechanical and biological filtration; all the other media, such as carbon and zeolite, are bolt-on goodies that may or may not be of any value. Personally, I consider a good quality set of sponges to be among the best value filter media for a freshwater tank, being a nice balance between cost, effectiveness, and longevity.
Filter aids and filter tonics
One final note. There are various tonics and potions available that do things like remove silt (e.g., Interpet "Filter Aid") or boost the effectiveness of the filter bacteria each time you do a water change (e.g., Interpet "Filter Start"). Silt removers are very useful if you have just added pond soil or sand to a tank, but otherwise serve no real purpose. Note that to get the best use from these filter aids, you need a fine medium such as wool in the filter. As for the bacterial supplements, they at least don't do any harm, and if you want to use them, then go ahead. They certainly aren't critical though.
Hope this is useful!
Edited by nmonks, 12 May 2006 - 10:30 AM.