Media is the plural of medium and in aquarist terms relates to the passage of water through a suitable material, to have an affect on, or change to the water. In other words, filtration, and without it waste matter would quickly build up and foul the water, turning it into a deadly cocktail of chemicals.



Media is classed into three categories, Mechanical - Biological - Chemical, each with its own job to do. Mechanical filter materials like filter floss (a synthetic wool), or foam sponge, are very good at removing floating debris from the water, a term referred to as polishing.

As well as making the water clear, its primary function, when used in multi-chambered filters, is to prevent the biological media from clogging. Biological action will occur to a degree on mechanical media as water passes through it. Because of the nature of their job mechanical media require regular cleaning.

Biological or bacterial media should have lots of surface area to perform well. This is where the microscopic bacteria live, and they take up residence in the millions of tiny holes made available to them in the media. Foam (specifically for aquarium use, non toxic) can be used in this application because of its large surface area, and water easily passes through it supplying the bacterial colony with fresh oxygen and nutrients, indeed, many filters rely on foam as the main medium.

There are many products available claiming very high surface areas, such as biotic rock, ceramic media and sintered glass for example. These products are extremely porous, hence the high surface area. If they were to be flattened out they wouldn't work very well, but the surface area would be huge.

Some claim to have surface areas the size of a doubles tennis court from a one-litre pack. Bacteria are very tiny and will colonize anything they can cling to, so enough of anything will act as a medium, even the gravel in your tank.

Chemical media is the final stage of filtration, whereby chemical media will change the waters qualities by reaction, and adsorption. Carbon is the most popular chemical filter medium, and is very good at removing pollutants from freshwater and marine aquaria, or rainwater before it is used in your aquarium.

Unlike mechanical media, which can be washed and re-used, or biological media that will last as long as it is colonised by bacteria, carbon will become saturated and have to be renewed. Failure to renew carbon can lead to the pollutants it's collected breaking down and being released back into the water.

Phosphates and nitrates can be removed with the use of various resin-based media, while zeolite, another popular chemical filtration media works by removing ammonia from fresh water aquaria, and releasing less harmful sodium. Unlike carbon, that must be discarded when used up, washing zeolite in salty water can recharge it; this will flush out the ammonia and replace it with more sodium.

In order to keep the nitrification process working efficiently, good aquarium hygiene must be observed, which means regular maintenance, keeping gravel clean, removing debris and uneaten food. Above all do not overstock or overfeed, this is inviting problems.

The aquarium is an enclosed environment, relying on artificial means to survive, and if the media becomes clogged it will dramatically reduce the surface area available for colonisation, and significantly slow down ammonia and nitrite consumption, which in turn will lead to stress in your fishes and outbreaks of disease.