The filter media does not need to be of any particular type or shape (for example, a cartridge). Unless the manufacturer’s instructions state otherwise, the substances listed below can be used in any type of container filter.
• Calcium carbonate. Used in the form of coral sand, coral chips (crushed corals) or limestone rubble as a chemical that increases water hardness and increases or buffers the pH. It can also act as a mechanical and biological filter. This filtering agent should be rinsed with water before use and thus cleaned of dust. It can be periodically washed and used again.
• Activated carbon. It is used as a means of removing some medicines from water (especially organic dyes such as methylene blue) and dissolved heavy metals. Contrary to popular belief, it does not remove the products of the nitrogen cycle – ammonia, nitrites and nitrates. It can also act as a filtering material for mechanical and biological cleaning. Coal works by adsorption – pollutants are collected on its surface. The amount of substance that it can adsorb is limited, so it needs to be replaced regularly. Activated carbon is charcoal that has undergone a special treatment in order to increase its porosity (and consequently, a large surface area), and thus its ability to adsorb. Before use, the coal should be rinsed in water to remove dust.
• Ceramics. Many ceramic products, especially hollow tubes, can serve as filtering material for coarse filtration. They act as a mechanical and biological filter medium and are commonly used in the first section of multisectional filters, since they are not so easily clogged and freely allow the flow of water. They can be washed and reused.
• Synthetic threads. Nowadays nylon yarn is often considered an old-fashioned tool, but it is still an inexpensive and very effective filtering material that allows mechanical and biological filtration. It can be repeatedly washed and reused.
• Sponge. Another cheap and very effective mechanical and biological filter material. Usually sold in the form of cartridges attached to certain filters. The sponge can be washed and reused many times, but in the end it loses its elasticity and needs to be replaced. It is very important to use only a special sponge for filters – its other types may be toxic to fish.
• Gravel. Aquarium gravel of any type (but not sand that is too easily clogged), should be washed before use, can be used for mechanical and biological filtration. It can also act as a chemically active filter medium, unless it is special gravel that is inert to water and does not increase the hardness of the water. It can be washed and reused indefinitely.
• Peat. Usually used as a chemical medium to lower the pH. It can also act mechanically and biologically. It gives water the color of tea (seemingly quite natural to many fish originating from reservoirs with acidic water). Peat should be used only in a nylon bag (a nylon stocking is ideal for this), since otherwise it will most likely penetrate the aquarium water. Before using the bag of peat should be rinsed in water to wash off the dust. You can buy a special peat for aquariums, but a completely acceptable alternative is garden peat with no additives, and it is also much cheaper. (If this raw material (as well as many other gardening orientations) was treated with special means (fertilizers, herbicides, insecticides, inhibitors, etc.), such savings can lead to big troubles).
• Sintered glass. This is a glass that has undergone a special treatment, as a result of which it becomes extremely porous. This results in a very large surface area on which colonies of bacteria settle. Thus a very effective biological environment is obtained. If it is used for mechanical filtration, it quickly becomes clogged. Therefore, even before the water reaches the glass, it must pass, for example, through a layer of sponge to remove from it the bulk of the mechanical suspension. Sintered glass is an expensive material, but because of its high biological efficiency, its relatively small amount is equivalent to a much larger number of filaments, sponges, gravel, etc.
• Zeolite is a naturally occurring ion exchange resin that neutralizes ammonia and is used primarily as a chemical filter material. It can also act mechanically and biologically. Zeolite has a limited service life, but it can be restored by soaking for 24 hours in a strong brine. Rinse with fresh water before reuse. Zeolite should not be used as a daily biological filter filler – otherwise, when it is fully developed, an ammonia crisis will occur in the aquarium. It helps to deal with temporary problems associated with ammonia, for example, in hospital aquariums, where medicine is used that is harmful to biological filters, as well as in containers for transporting fish, so that the fish during long trips are not poisoned by their own waste.