There's no real answer to the question of whether carbon is good or bad for a discus aquarium. It has been the subject of many flames and this page is here to provide some information about what carbon actually does.
It is a partial reprint of an article entitled "Myths of Aquarium Filtration and Water Chemistry". It first appeared in the 1995 issue of AQUARIUM USA, and is written by Timothy A Hovanec, who has over 30 years of experience as an aquarist. He also has extensive professional expertise in aquatic system design, water quality research and aquaculture. He is currently the director of aquatic research for Marineland Aquarium Products and is a PhD candidate in biology at the University of California. Mr. Hovanec has given explicit permission for this material to be used in the DISCUS-L mailing list and on this web page. It may not be used in any other manner without permission from Mr. Hovanec.
There is a perception among hobbyists that all activated carbons are the same. Assuming this is true, one can simply shop for carbon by paying attention to price. Paradoxically, a smaller group of aquarists believes that the most expensive carbon is the best. Both of these assumptions are untrue.
There are four common base materials used to make activated carbon. These base materials are bituminous coal, lignite, peat, and coconut shells. Although these are listed here in order of most to least effective, even within a specific material group there can be differences.
Activated carbons made from bituminous coal are the most effective for aquarium water filtration. They contain a wide range of pore sizes and are thus able to remove a wide range of organic pollutants. The pore size is important because it determines the size of the organic compounds the carbon can remove.
A carbon can have a very large surface area, but that area can be mostly unusable if the organic compounds to be removed are too large to fit into the pores. This is the case with coconut shell carbon. The structure of coconut is such that the activated carbon it forms has a tremendous number of very small pores, while most of the organics in the water are larger than these pores and will not fit into them. This eliminates any benefit of a large amount of surface area.
Lignite coal produces carbons with the opposite problem from coconut carbons. Lignitel carbons have pores large enough for common organic molecules, but this results in a reduced total surface area. To remove the same amount of organics, a lot more lignite carbon must be used compared to bituminous coal carbon.
Do not buy a carbon just based on surface area claims. Read the label and determine the type of base material. If it is not listed, contact the manufacturer. Don't pay extra for pre-washed carbon or "marine-grade" carbons. There is no carbon specially made for saltwater. Any quality carbon will work in both freshwater and saltwater aquariums.
Price is no indicator of carbon quality. While better carbons are more expensive, the most expensive carbons are not necessarily worth the extra cost. Size is also not an indicator of effectiveness. A small-granule carbon does not have more surface area (more pores) than a large-granule carbon. The reason for this is that the internal pore surface area, not the external area of the granule, determines the total surface area. Smaller carbon will work faster than larger carbon, but will not remove any more pollutants.
All carbons should be rinsed well before being placed in the filter system. Keep an eye on the water for any yellowish tint that indicates that the carbon needs to be replaced.
Many people do not use carbon because they beleive it will remove beneficial trace elements. It is true that activated carbon will remove certain trace elements under certain conditions. However, this statement is much different from the general proclimation that carbon removes trace elements, which implies all trace elements are removed in all conditions.
The ability of carbon to remove a particular trace element depends upon the pH of the water, the concentration of the element, whether the element is chelated or not, and the "state" of the element. Some elements exist in many forms (states) in water. Iron, for example, exists in either the ferric or ferrous state. Carbon does not adsorb iron in the ferrous state and only moderately in the ferric state.
The pH is important because it determines the solubility of the elements. Studies have shown that the pH must be below 4 or above 10 for carbon to adsorb many elements. Table 1 shows the adsorption potential of many substances. While the chart is not definitive, it gives a good idea of the types of substances adsorbed by carbon.
| High to Very Good | Good to Moderate | Fair | Low to None |
|---|---|---|---|
| Antimony | Acetic Acid | Copper (if complexed) | Alkalinity |
| Arsenic | Cobalt | Iron (as Fe3+) | Ammonia |
| Bismuth | Detergents | Lead | Barium |
| Bleach | Hydrogen Sulfide | Nickel | Beryllium |
| Chloramines | Mercury | Titanium | Cadmium |
| Chlorine | Ozone | Vanadium | Carbon Dioxide |
| Chromium | Potassium Permanganate | Copper | |
| Colors | Silver | Hardness | |
| Dyes | Soap | Iron (as Fe2+) | |
| Gold | Solvents | Lime | |
| Hydrogen Peroxide | Vinegar | Manganese | |
| Insecticides | Zirconium | Molybdenum | |
| Monochloramine | Nitrates | ||
| Odors | Selenium | ||
| Pesticides | Tungsten | ||
| Phenols | Zinc | ||
| Tin | |||
| Trihalomethanes |