Activated carbon for water purification.
Continued…
How to choose good activated carbon? It is best to purchase special carbon designed for aquariums. As a filler for carbon contactors, domestic birch charcoal of the BAU brand can be successfully used. Try to pay attention to the following characteristics: • Particles should be small – about 1-2 mm in size (although some companies make them larger); • Particles should be hard and not shiny in appearance. If the particles are soft and shiny, then this is most likely ordinary, non-activated carbon; • Carbon should be light – this means it has many pores. Particles should repel water, and the pores should be filled with air – at first the carbon floats, and if you listen closely, you can hear air coming out of the pores; if the carbon is heavy and sinks, it is not of very good quality; • Carbon should not contain a large amount of dust. Before use, it is advisable to rinse the carbon in clean running water to remove coal dust, and then boil it, especially if a decrease in pH was noted in the aquarium after its use.
Many manufacturers offer so-called cartridge filters for their products: their outer shell consists of mechanical filter material (perlon wool), and the 
inner cartridge, on the contrary, is empty and intended for filling with carbon, peat, or another similar filtering mass. Some filter models also have a small compartment for loose filler or synthetic winterizer. Therefore, it is better to purchase an internal filter with at least one additional section for possible filling with carbon.
How much carbon should be used? This depends on the purposes for which you are using it. For example, to keep the water crystal clear, it can be used in large quantities and changed more often. Calculating the amount of carbon is quite simple: 1 gram of carbon should account for 1 liter of fresh aquarium water. On the other hand, it can be used in moderate quantities (about 5-20 g per 50 liters) and changed when the water acquires a yellowish tint. On average, good aquarium carbon is able to absorb organics (of course, at a reasonable amount in the water) within 2-4 months. But it is better to change it more often.
More precise recommendations cannot be given – everything depends on specific conditions. If you have unlimited supplies of carbon, you can change it frequently; if the water has acquired a yellowish tint (and this bothers you), it is time to change the carbon. To determine if the water has acquired a yellowish tint, use a strip of white plastic, part of which is painted in a yellow-brown color (for example, with a permanent marker). If, when immersed in water, the border between white and yellow is not visible, it is time to change the carbon. To check if the activated carbon is still working, you can add a little food coloring to the aquarium in the evening – if the color does not disappear by morning, it is time to replace the old activated carbon with a new one. If you filtered out an antibiotic after treating a disease in the aquarium, you need to change the carbon.
A few words about carbon regeneration. It is impossible to completely regenerate it at home. If the carbon was not used for very long, partial regeneration can be done once. There are three methods: a) the carbon is calcined on a metal baking sheet, and some of the organic substances burn out; b) baking soda is added to the washed carbon (1.5 tablespoons per 1 liter of carbon), filled with boiling water, and boiled for 15 minutes with constant stirring, then thoroughly rinsed under running water; c) the activated carbon must be boiled in distilled water for an hour, changing the water twice.
The effectiveness of such regeneration is doubtful and makes no sense for old carbon – it is better to take a new one. Some filtering substances (for example, zeolite) can be regenerated, while others, most often chemical filtering substances (for example, peat or carbon), need regular replacement. In the filter, activated carbon and other fillers, including ceramics, are best kept in separate mesh bags – then it will be more convenient to pull them out and wash them.
Now a few words on why you should not overuse carbon in the aquarium. Beginners usually use it in the filter, believing that it is a panacea for all problems. Carbon does not remove nitrates and phosphates, which are one of the main causes of algae in the aquarium. On the contrary, low-quality carbon will release phosphates. On the other hand, carbon will absorb fertilizers necessary for plants, such as iron.
Daily use of this procedure can be disastrous because it removes dissolved organic carbon, which not only prevents metal poisoning in fish but also provides plants with CO2 and nutrients. Of course, once bacteria settle on it, they will process ammonia through biological filtration. But using it for such purposes is expensive. By making the water crystal clear, carbon masks the need for a water change – the aquarist will assume the water does not need changing while nitrates are off the charts.
With regular water changes, there should not be much organic matter in the aquarium, and if there is a lot, you need to look for the cause – overfeeding the fish in the aquarium, etc. The yellowness of the water – in general, it does not particularly interfere within reasonable limits; even on the contrary, the aquarium looks more natural with slightly yellowish water. Fish are not bothered by this at all. In fact, chemical cleaning is not a mandatory link in every aquarium. But occasionally fish still get sick, and medications have to be added to the water. And after a course of treatment, it is absolutely necessary to remove the medicine residues from the aquarium, and carbon is simply irreplaceable here. It is completely unreasonable to put peat in the filter to soften hard water, which releases organics that give the water a yellowish color, and along with it carbon, which will absorb this organics.
To remove unwanted organic pollutants, aquarists usually follow a very simplified scheme: they put an adsorption, or in other words, chemical filter with activated carbon or other substances called sorbents (natural or synthetic) into the aquarium. At the same time, they believe that by periodically changing the filler of such a filter, they solve all problems with associated organics. Unfortunately, this is not always and not entirely true.
The most important thing when using sorbents is to correctly determine the replacement schedule. At best, they simply stop performing their functions, since the sorbing surfaces can no longer accept pollutants, and at worst, spontaneous desorption occurs – a sudden release of everything accumulated in them into the aquarium water. One thing is certain: stress for the aquarium inhabitants cannot be avoided. Particularly sensitive hydrobionts experience stress even at the slightest glitch, which is why aquarists often install two carbon contactors, which they change in turn, so that there is no sharp change in concentration due to the better sorbing capacity of the fresh active substance.
The process of ozonation of aquarium water performs two tasks simultaneously: it disinfects the water and oxidizes some organic substances. The negative action of ozone is its reactions with iron, manganese, and other vital substances, which are then removed from the water in the form of insoluble oxides; therefore, when using ozone purification, it is especially important to choose the required dose, and then filter the water through a carbon filter. Water from this vessel should be supplied to the aquarium through a carbon filter, which is used to remove traces of dissolved ozone, preventing them from reaching the fish. Even ozonated water passed through activated carbon retains almost all of its bactericidal properties.
As a result, we can recommend the periodic use of carbon – for example, before introducing antibiotics and other medical preparations into the aquarium. This removes organic matter 
that reduces the effectiveness of medicines. Similarly, carbon can be used to remove medications after treatment. After treatment, free copper from the applied medications must be removed by partial water changes, and then filtration through activated carbon is necessary. Water can be colored by certain medications (for example, acriflavine and methylene blue). This type of water discoloration can be eliminated after the course of treatment by filtering the water through activated carbon. Often colored gravel also stains the water in various colors. If you have pink gravel and the water has acquired a reddish tint, regular activated carbon will help you.
The chlorine content in tap water can vary depending on the area, season, and other conditions. A reliable method is the adsorption method – running chlorinated tap water through activated carbon, chlorine is adsorbed on it, and the water is freed from the toxic component. Charcoal can be added by including it in nutrient balls made of clay and peat. With episodic use of carbon, make sure that anaerobic conditions do not form in the pores.
Ion-exchange resins. Zeolites, Cationites, Anionites, Water deionization.
Activated carbon for water purification. 
