Synthetic filter wool is used primarily as a mechanical filter medium. Whilst sponges and ceramic chips can also be used thus, filter wool traps finer particles more effectively than either of them. The more tightly it is packed into the filter, the finer the particles it will collect, making it ideal for 'polishing' aquarium water by straining out things like silt and mulm. Because it is inexpensive, many aquarists simply replace filter wool once it gets dirty, but it can be reused. The trick is to rinse it off every couple of weeks, before the filter wool gets too matted with dirt. Once that happens, it is very difficult to clean satisfactorily.

Filter wool can also be used as a biological filter medium. It is one of the best media to use in box filters, because it is easy to shape the mass of wool to fill the box nicely. Coarser media, like ceramic chips, tend to leave big gaps around the edges through which water will pass without being cleaned. Air-powered box filters are especially useful in breeding tanks.

Synthetic sponges are included with most aquarium filters, ranging from simple clip-on air-powered devices through to the big canister filters. Sponges are wonderfully durable compared with filter wool, and if properly looked after, can last for many years. Because sponges are usually designed for a specific filter, they fit the inside of the filter very neatly, providing maximum efficiency. On the downside, sponges are much more 'open' than filter wool, and so aren't nearly so good at removing silt and other fine particles. Neither do they provide quite the same surface area per unit volume as the best ceramic media, making them slightly inferior as far as biological filtration goes. Sponges are very much 'jack of all trades, but master of none' when it comes to filtration.

While simple filters may only have a single sponge, most filters will include a variety of different sponges, each meant to serve a certain function in the system. One popular all-in-one series of aquaria comes with filters containing no fewer than three different types of sponge: coarse sponges for mechanical filtration, fine sponges for biological filtration, and carbon-impregnated sponges for chemical filtration. The carbon sponge has a definite lifespan, and after a few weeks will need to be replaced. Regardless of the type of filter system, it is important that sponges are cleaned regularly if they are to remain useful. Squeezing them a few times in a bucket of aquarium water will usually do the trick, but periodically you may want to give the sponges a more thorough clean. Washing under running water will bring an old sponge back to near-pristine condition, but in the process, virtually all the filter bacteria on it will be killed. It is therefore important to only do this to some of the sponges at any one time, leaving the others where they are, so that they can re-seed the clean sponge with filter bacteria afterwards.

Ceramic tubes of various kinds are widely considered the best substrate for biological filtration. This is because the ceramic tubes have a finely pitted surface area that allows for a vast population of nitrifying bacteria. Ceramic tubes won't become too tightly packed, either, making sure that there is a good water flow around each piece. On the other hand, this does mean that you cannot wedge so much of the stuff into a small space, so ceramic media works best in big filters where there is plenty of space. Being rigid, it can also be used in wet-and-dry filters and trickle filters, where sponges or filter wool wouldn't work well, if at all. All of the major manufacturers of filters produce their own brand of ceramic media, and whilst each purports to have its own advantages, they all work well.

Though not able to trap such fine particles as filter wool, ceramic media can also be used as a mechanical filter medium. Many manufacturers produce their ceramic media in two grades, a coarser media for mechanical filtration, and a finer one for biological filtration. The coarser media is a bit cheaper than the finer stuff, but both are relatively expensive compared with filter wool or sponges. Ceramic media will last for many years though, making it very good value over the long term. As with all filter media, maintenance is important. Periodically, a quarter to a third of the media should be removed and replaced with some fresh media. The old media can then be vigorously cleaned in warm, soapy water and then left to soak in a bucket of water for day or two. Rinse and soak a few more times to get rid of any traces of the soap, let the media dry, and then pack away somewhere it won't get dusty. Next time you need to do a change of filter media, this now-spotlessly clean filter media can be pressed into service, and some of the dirty media taken from the filter and cleaned in the same way.

Undergravel filters are among the most effective aquarium filters, even if they are somewhat out of fashion these days. Undergravel filters do have many shortcomings, it is true: undergravel filters inhibit plant growth by oxidising mineral nutrients; they require regular cleaning if they are to remain effective; and they restrict substrate choice to a uniform layer of gravel around 8-10 cm or so in depth largely unobstructed by big pieces of bogwood or stone. Undergravel filters cannot be used with special substrates such as laterite or aquarium soil either, limiting their usefulness in tanks where plants are important.

These limitations aside, a good undergravel filter is astonishingly effective and contains a vast surface area on which the nitrifying bacteria can grow, and compared with canister filters, maintenance is relatively straightforward. On a weekly basis, the accumulated detritus on the top of the gravel bed (known as mulm) should be siphoned away, and every month or two the gravel should be more vigorously stirred and cleaned.

In marine aquaria coral sand is often used as a biological filtration medium, but in freshwater tanks it is more normally used as a chemical filtration medium. As water passes through the coral sand, the coral sand slowly dissolves, raising the pH and hardness levels. This is very useful in tanks containing species such as Rift Valley cichlids, Central American cichlids, livebearers, and brackish water fishes, all of which appreciate hard, alkaline water. Coral sand can either be incorporated into an undergravel filter alongside plain gravel, or installed in one of the compartments in a canister filter.

One limitation to coral sand is that it only buffers the water when it is clean. Once installed in a filter it will accumulate a coating of bacteria and algae. These insulate the grain of coral sand from the water, preventing the movement of the pH and hardness modifying chemicals from the sand to the water. So, for maximum effectiveness, coral sand should be cleaned or replaced each time the filter is cleaned.

During the early days of the hobby, received wisdom was that the old water in an aquarium was somehow good for the fish. It was believed that water changes should be performed only infrequently, for fear of disturbing the balance of the aquarium. Old aquarium water wasn't very attractive though, having a yellow tint formed by various dissolved organic compounds. The solution was to incorporate carbon into the filter, as this would remove the organic compounds very effectively.

Modern fishkeeping recognises the value of frequent water changes, with weekly changes of at least 20% being generally recommended. When this is done, the water doesn't turn yellow, and so the job of carbon no longer exists. Nonetheless, carbon is still sold in aquarium shops, and so many aquarists assume it serves a purpose. It doesn't, and you don't need to buy it. If anything, carbon is counterproductive in most aquaria. Because it removes organic compounds indiscriminately, it will take out not just bad stuff but also medications. Anyone treating fish for Whitespot or any other ailment must remove carbon from the filter before dosing the tank. The best approach is simply not to use carbon at all.

Like carbon, ammonia remove (Zeolite) is widely sold and thus assumed to be useful, but in fact it isn't required in most aquaria. A healthy biological filter does just as good a job, but unlike Zeolite, a biological filter doesn't become 'saturated'. Zeolite can only absorb a certain amount of ammonium; after that, it stops working, which puts your fish at risk. If Zeolite is used instead of a biological filter, it must be replaced regularly, and a careful check made on ammonium levels in the tank using the appropriate test kit. Once saturated, it is usually possible to recharge spent Zeolite by soaking it in salty water for a few days. For this reason, buy at least twice as much Zeolite as you need, so you can have one batch in the aquarium while another batch is being recharged.

Zeolite is most useful in tanks that need to be set up quickly. A hospital tank, for example, may need to be pressed into service without time to mature a biological filter. Moreover, because the hospital tank needs to be 'clean', moving mature filter bacteria from the main aquarium would carry the risk of transferring more of whatever parasite or bacteria you were attempting to treat. Being a chemical rather than a biological medium, Zeolite is unaffected by antibiotics and other strong medications that can harm filter bacteria. Zeolite works very well in tanks such as these, where stocking density will be low, so the Zeolite will not be saturated particularly quickly.

Ordinary peat has been used widely in the past as a way to condition aquarium water, softening it and lowering the pH. This is particularly appreciated by tetras, South American cichlids, and many killifish, which may live in harder water, but won't breed in it. Peat granulate is a concentrated form of peat that is especially convenient and easy to use, taking up only a little space in a canister filter. It is perhaps the most widely sold form of peat for use in filters. When used properly, peat gives the water a lovely dark tint that dramatically enhances the luminous colours of fishes such as neons, cardinals, Glowlight tetras, and rasboras.

Using peat granulate is tricky because it can cause quite rapid changes in pH and hardness. Always read the box carefully, follow the instructions, and err on the side of using too little rather than too much. As water gets softer, its ability to resist changes in pH decreases, and so soft water is often chemically very unstable. Whilst some fish can tolerate quite a low pH level, most aquarium fish only want slightly acidic conditions (around pH 6.5), so it is important to measure pH regularly.