Be clear about carbon


There are many uses for carbon, especially in the fishkeeping hobby, yet it's often misunderstood, writes Nathan Hill. Here's your chance to get to know this chemical element like the back of your hand.

Look in your tank, it’ll be heaving with carbon. Look at the back of your hand, that’s carbon heavy, too. In fact, carbon permeates all aspects of our lives, from peanuts to pencils. It’s only right we should find many uses for it, and a lot of those uses are aquarium-based.

Carbon is, in its simplest form, the chemical element C. If you know your periodic table, you’ll recognise it as the sixth element down the line. It’s naturally occurring, bountiful and incredibly useful.

You’ll already be deliberately using carbon in some form, whether you like it or not. It makes up a vital ingredient in food, such as protein.

Your fish will be breathing it out through their gills as carbon dioxide. Your plants will be guzzling it up as food to make sugars — C6H12O6. Carbon is absolutely inescapable.

Filter carbon

For the best part, most of us will know carbon as the black granular media we cram into our filters. You might have pads impregnated with it, or you might have a pack of it loose, ready to be loaded up into a mesh bag and dropped in the tank somewhere. That’s all well and good, but what does this kind of carbon do? How does it work?

Carbon works on adsorption (not absorption), which is a way of saying it is incredibly sticky to certain chemicals. Once certain chemicals come in to contact with carbon, they are grabbed by it and retained — at least for a while.

This is why the surface area of carbon is all-important. The more it has, the more waste it can hang on to. This is where activated carbon has the advantage over regular carbon, in that the activation process massively increases the number of tiny pores in it.

Carbon is excellent at removing organic chemicals from the water. Now, don’t confuse organic chemicals with the three usual suspects: ammonia, nitrite and nitrate. These three are inorganic compounds, and carbon has no interest in them. If you’re adding carbon to deal with an ammonia spike then you’re barking up the wrong tree.

The two chemicals that carbon excels at pulling from tanks are phenols and tannins. These are produced from a variety of sources, including the metabolic waste of fish, leaching from wood, the breakdown and decomposition of faeces, plants and uneaten food, and more. You’ll recognise them as chemicals that discolour the water, giving a yellow or brown tint, and in the case of phenols, emitting a 'tanky' smell as well.

In this respect, carbon is indispensible in polishing the water and keeping it gin clear. For marine keepers, those phenols can play havoc with marine invertebrate growth, where for freshwater they’re more likely to build up and cause eventual malaise in the fish.

It’s essential to get carbon in the right place in the filter as well. If it is placed too early then it runs the risk of becoming biologically active; that is to say that it will start to get smothered in nitrifying bacteria. If that happens, you lose surface area on the carbon and it becomes redundant. Even worse, when it comes to changing it, you’ll be pulling out a huge glut of your biological filter with it!

The carbon should always be a polishing agent at the end of the filtration process, never at the start. If you have a canister then you want to go foams (mechanical filters) then biological media and then carbon, not the other way around.

How much you need to use varies, based on your tank, the inhabitants and the quality of the carbon. The best carbon you can get has the highest surface area to volume ratio.

If faced with a choice of a few varieties, opt to buy the one that boasts the most surface area compared to its weight. Some brands will happily advertise their surface area on the packet, and this is a good sign that they know what they’re talking about.

Carbon as filter food

In marine tanks, carbon is frequently utilised as a way of encouraging and feeding the growth of anaerobic bacteria.

Anaerobic, heterotrophic bacteria — the kind that guzzle nitrates when there’s no oxygen around — require a carbon source in order to function, unlike the autotrophic bacteria in our filters that convert ammonia and nitrite. That carbon can take a few forms. Some newer products are appearing that use a starch as the carbon source (and with excellent results, I add).

The traditional method is to add a liquid carbon, namely alcohol, to the tank. The alcohol we have in drinks like vodka is ethanol — C2H5OH — which is just right for the bacteria to use. It might seem odd, but, yes, adding the correct amount of vodka to a marine tank with plenty of live rock will reduce both nitrates and phosphates. Different aquarists have had successes with different rates of vodka dosing, but methods that have worked have involved between 0.1 and 0.9ml of 40% alcohol vodka per 100 litres to a tank daily.

Of course, vodka isn’t the only way to get carbon into a tank, but it has given good results so far. Some aquarists have experimented with other ‘everyday’ sources such as vinegar, rice and sugar. The angle here is that these methods introduce carbon to the water column. The catch with that is that there’s every chance an overdose will poison fish. Uh oh.

That doesn’t spell the end for carbon dosing. In the wake of dangers presented by liquid carbons, the market has increasingly steered towards solid carbon sources, namely starches and other carbohydrates, or ‘dry vodka’ dosing. Don’t be deceived by the 'vodka' part in this latter claim, as vodka is not an ingredient at all; it’s just a direct reference to the fact that it’s doing a similar job to vodka dosing methods.

Solid carbon reverses the roles a tad, and rather than bringing carbon to the bacteria (who usually live in rocks or substrate), this method entices bacteria to grow directly on the carbon source. Most solid carbon methods employ the use of a reactor, which is a large tube through which the water from the tank passes over the carbon filter medium. This has the effect of tumbling the media and keeping it in suspension, and as the carbon then develops a film on its surface — made up of nitrate and phosphate-guzzling, carbon-hungry bacteria — this tumbling action knocks the excess biofilm off.

The film now comprised of bacteria and fixed waste, is then either removed through the action of a protein skimmer or is even consumed by corals and other invertebrates in the tank.

Flow rate is essential when it comes to solid carbon dosing. Too fast, and the filter doesn’t become established; too slow, and everything gets smothered in biofilm. Solid carbon is still a fast growing part of the marine fishkeeping hobby.

Carbon as plant food

Carbon is essential for plant growth, but it needs to be in the right form for plants to uptake.

I’m sure most of us are well aware of what photosynthesis is, but to briefly recap it’s the action of plants that utilises water, light and carbon dioxide in order to create carbohydrates and oxygen.

In the aquarium, carbon is added in one of a couple of ways. The preferable method for many is to add carbon dioxide to the water, through the medium of tiny bubbles until it forms a solution in the water. As humans, we guzzle down the stuff as the main ingredient in fizzy drinks: carbonated water. One alternative name for carbonated water is carbonic acid, and that’s one of the reasons why soft drinks are so acidic: because you’re literally drinking acid. Add too much to the tank, and it’ll drop to dangerously low pH levels, which is why a thorough understanding of the use of CO2 in planted tanks is essential and why so many 'budget' CO2 kits can, in fact, be wildly dangerous.

In absence of adequate carbonic acid, plants in tanks will try to find their carbon from elsewhere. Calcium carbonate, the chemical responsible for the alkalinity and carbonate hardness in your tank, is sometimes forcefully secured as an alternative source and can result in faint chalky deposits on plants.

Carbon can also be added in liquid form, and the current vogue of plant growers is the use of glutaraldehyde, CH2(CH2CHO)2. This form of carbon is not without its risks. In fact, in its everyday use, it’s a steriliser for dental gear, a wart cure and a fixer for tanning leather. Overdose it and it’ll ravage fish and invertebrates.

Carbon as dechlorinator

One area where carbon is outstanding is in the removal of chlorine, chloramine and some heavy metals. With that in mind, it’s a pristine choice to treat any incoming tapwater before use with fish, and it can be considerably more economical than using liquid dechlorinators (some of which don’t even remove chloramine!)

To put into context, a typical 500ml bottle of dechlorinator will cost me about £9 and treat around 19,000 litres. Ten bottles will treat about 190,000 litres and cost me £90. A typical carbon dechlorinator treating 350,000 litres can be picked up for around £90 to £115. That’s a big saving…

A carbon dechlorinator goes inline on your water supply. Some need to be permanently installed, which is great for domestic supplies as well as tanks or, if you prefer, you can buy units that simply go inline with hosing.

You pay your money and take your choice.

The lifespan of carbon

Due to its adsorptive nature, carbon eventually exhausts itself when its entire surface is coated in waste. Anyone that tells you an exact lifespan for carbon is a barefaced liar, as each and every tank will have a different volume of waste inside it. In some cases, the carbon might be exhausted within three or four weeks. Most users will have it in their tank for up to six week stints at a time, after which time we generally accept that it can start to leach some of what it has taken out back in to the water.

In that time, you’ll find that carbon is very good at removing chlorine; chloramine; phenols; dyes; some heavy metals such as tin, mercury and iron; and fish medications. Indeed, that’s one of the best reasons to have carbon on hand: to whisk out any accidental overdoses or to remove residual medication after a disease treatment.

PFK answers your carbon FAQ's

Where is carbon from?

Carbon comes from lots of different sources. On a universal level, it is formed in the death throes of exploding stars and blasted out into space during their final cough.

For aquaria, granular carbon can come from animal bones, peat, coconut shells, wood and bituminous or lignite coal. The source can have a big effect on just how good a carbon is in action. Effectively, all carbon is just charcoal taken to the next level by cooking it.

To create carbon, these ingredients need to be heated to thousands of degrees centigrade, which gets rid of the impurities and chemicals you don’t want, as well as increasing the surface area. Surface area is essential, as the more you have, the more your carbon can interact with water and the more chemicals it can remove.

For filters, it’s generally accepted that carbon made from bituminous coal is the best. In fact, it’s the choice of water purifiers the world over, and if you have a BRITA filter, then you can bet your boots you’ll have this kind of carbon inside it.

Carbon dioxide is sometimes harvested from furnaces, breweries and other ‘recapture’ sources but is also the by-product of burning coke. It can even be made with certain chemical reactions, when acids meet base substances. The kinds of carbon found in solid carbon pellets are often specifically processed starches created in laboratory calibre conditions.

Can carbon be reactivated in the oven?

No. The myth goes that you can place your old carbon in the oven, turn it up to full heat for a few hours and the carbon refreshes. Unless you have an oven that goes up to 2,000°C or more and have access to facilitators like acid and oxygen, then it’ll do nothing. Besides, who can afford to run an oven like that with today’s energy costs?

Didn’t you say it removes organics? What about trace elements?

Admittedly, it’s not just organic chemicals that carbon removes, and as such it has been implicated as a cause of problems to some fish, even as the cause of a disease.

It’s true that carbon is unselective in what it removes. But because of this, it’s had the finger pointed at it for removing trace elements and even micronutrients that fish need. One spurious extrapolation of this idea is that carbon might even cause head and lateral line erosion (HLLE) or hole in the head (HITH) disease. This was touted as mantra by many aquarists for years until further research suggested that it might instead be certain ferrous chemicals used in phosphate removers that were causing the problem.

Either way, the amount of trace elements that carbon can be accused of removing leaves it off the hook when it comes to causing illness.

Do I still need to perform water changes?

Oh yes. Carbon is not a water change in a bag. Despite it pulling out some chemicals, especially dissolved organics, you will still have a build-up of nitrate and a depletion of minerals that need replacing.

Does it leach phosphate?

Yes, the one downside to activated carbon is that it will increase phosphate levels in the tank, which in turn is implicated in the boosting of many kinds of algae in everyday aquaria.

Really cheap, low quality carbon, heaving with ash, will even boost pH in your tank, sometimes dangerously so. As luck would have it, you’ll usually find that these cheaper brands will also be the worst for leaching phosphate, so with carbon it’s often a case of getting what you pay for. If it looks too cheap to be true, it probably is.

Organic or inorganic?

Whether a compound is organic or not rests heavily on carbon, though not without exceptions.

Most organic chemicals that we can think of usually involve carbon in tandem with hydrogen. So, proteins (carbon, hydrogen, oxygen and nitrogen) and sugars (carbon, hydrogen and oxygen) are among the many organic chemicals that we come across.

It’s not quite enough to class carbon as the single factor, however, and not everything carbon-based is organic. A diamond, for example, is pure carbon, but as inorganic as it gets. Carbon dioxide (carbon and oxygen) is another inorganic carbon form. There are even a couple of organic compounds that do not have the hydrogen bonding, yet they are still classed as organic.

It’s rare that you’ll come across the phrase organic or inorganic carbon in the hobby, but if you do, just know that what we’re referring to is the presence or otherwise of both carbon and hydrogen in a compound!