Dr. Timothy A. Hovanec
There is much written about the use of carbon in seawater aquaria. Some authors recommend against the use of carbon in seawater aquaria because carbon will remove trace elements. As with most situations in the aquarium hobby, the correct response to the claim that activated carbon removes trace elements from seawater is yes and no– mostly no. By definition, trace elements are those elements found in the ocean with a concentration of less than 1 ppb. These are elements such as cesium, chromium, cadmium, selenium, cobalt, silver, lead, tin, helium, lanthanum and cerium. For these elements, it has never been shown that they are really needed in the aquarium. In fact, adding some to the water would really be poisoning the tank. Further, these elements are rather insoluble at the pH of seawater, so activated carbon cannot remove them from the water. Thus the correct technical answer is that in the normal marine aquarium, activated carbon will not remove trace elements. However, if one wanted to remove some of the above listed trace elements from seawater with activated carbon it could be done by changing the pH and some other factors to get the elements in the soluble form but this would render the seawater uninhabitable.
So what elements might carbon removes from seawater that would be of interest to the marine hobbyist? These elements would be copper, iodine, iron and molybdenum which, by definition, are called minor elements. However, once again, how much of any of these elements carbon might remove depends upon the element’s solubility in seawater. Most of these elements are not in the soluble form in seawater and, as such, cannot be removed from the seawater by carbon. The bottom line is that activated carbon is going to have no effect on the majority of elements found in seawater.
Activated carbon should be part of every aquarium filtration system. This is another statement that some have disagreed with me about. However, personal experience with using activated carbon for years along with data collected in my laboratories supports my claim. Carbon has been used continuously in all types of freshwater tanks, saltwater fish-only tanks, reef tanks and large display systems in the labs and never has there been a problem.
The reasons given for not using carbon or not using it continuously include: the carbon causes some diseases, it removes beneficial elements/substances for the water, and eventually the carbon will de-adsorb everything killing the tank inhabitants. None of these reasons have ever been supported by experiments or tests. Rather, they are myths. And as myths, they circulate through the hobby growing a life of their own which becomes hard to kill.
The reason to use carbon is that it removes dissolved organic compound from the water. This has been shown experimentally. Removing the dissolved organics will increase the effectiveness of the biological filter. Again published studies have demonstrated this. Plus, removing the dissolved organics will make the water look cleaner because it will be clearer.
For most filters, GAC comes conveniently pre-packaged so that it can be easily removed from the filter and replaced. The most efficient method is to get as much of the water to go through the carbon on each pass through the filter. This can be done by spreading the carbon out in a thin layer rather than letting it pile up in a bag.
A problem with recommending the use of activated carbon is that there are no easy guides for answering the common questions regarding the use of carbon. Questions such as how much carbon is needed in a tank and how often it should be changed can only be answered definitively by using it in your aquarium. This is because every aquarium is different and how they are maintained is different. Because of the individual differences in tanks, these guidelines are, at best, educated guesses.
But general guidelines will work and those that I recommend are based on the observations of many aquaria run for long periods of time. Thus, for most situations, if your tank is not overcrowded or overfed, then changing the carbon once a month is sufficient. If you have big fish or a lot of fish, change the carbon every 2 or 3 weeks.
To get the most out of your carbon make sure to mechanically filter the water before it reaches the carbon, as this will keep the carbon from clogging with debris. A good practice is to rinse the carbon under water for a few seconds each week to get rid of particulate material. This will keep the water paths open and help you get the full benefit of the carbon.
Carbon is not a substitute for water changes. You cannot stop doing other maintenance chores. It is only one part of the filtration system. Most filters make using carbon easy and carbon is not that expensive. Carbon will help you get the most enjoyment from your aquarium and provide a healthy environment for your fish. Good fishkeeping!
©1993, Timothy A. Hovanec, Ph.D.
Originally published in Aquarium Fish Magazine, May. 1993