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Guide: Activated Carbon for Aquariums

Tony Callaghan 07/03/2018

Guide: Activated Carbon for Aquariums

Armed with knowledge you can save money on your activated carbon and still get excellent performance!

Using our knowledge on carbon, we have created a detailed guide that you can follow to get the best out of activated carbon for aquarium filtering!

Please note that we are not aquarium experts and are very happy to chat with you and review our guide if you think we have got something wrong.

Best Performing Activated Carbon for Aquariums

The best performing activated carbons tend to be those that are specifically formulated for aquarium use. However mid-range standard activated carbon products can also provide acceptable performance whilst having the additional benefit of lower costs.

Compare Activated Carbons Price and Performance

We have divided the activated carbons into three groups based on price and performance:

  1. Activated carbons specifically formulated for aquarium use:
    These are few and far between. You will pay a price premium. Look out for specific testing of the carbon when filtering aquarium water. Take care with branding and nice words that position “standard carbon” as "aquarium carbon". Beware of claims related to general carbon properties such as “high surface area”. (Read why below). Those in the know will openly explain what each grade does and does not do.

  2. Mid-range standard activated carbon products made in high volume for other applications, but sold for aquarium use:
    You get an okay performance and the benefit of lower prices. Many of these carbons appear as “re-bagged and branded” to be sold through the aquarium distributors. If possible, look for a long history of supplying a specific grade.  Watch for those who swap sources replacing carbon pellet type when they purchase from another supplier. If you're buying a standard grade – please read about the different sources, manufacturing process and quality. Your decision might not just be about price!

  3. Low grade carbons ‘sign’ posted for aquarium use:
    These might not be very useful at all in an aquarium.

The old adages around you get what you pay for and buyer beware apply. Armed with the activated carbon knowledge in this blog, we believe you can source good activated carbon at competitive prices.

SoilFixer Activated Carbon

Our water filtration charcoal granules are sieved to remove powder. The prices are per the shop prices for the 0-10mm grade, however we do not list the 2-8mm grade in the gardener's shop - you will need to email us to place an order.  

Having researched this blog, we believe we have the expertise to help prove we have something special. We hope to prove this via trials and testing. Until then – grab a bargain because we have launched the aquarium grade at a very competitive price point to some of the better known 'aquarium' brands.

Other Factors Affecting Your Decision On Which Carbon to Use

As well as price and performance, there may be many other reasons why you might have preferences and you may have questions such as:

  • Where does the carbon come from? What are the CO2 travel miles? (The majority of activated carbon comes from Asia and China). Even some “European” carbon is imported to the EEC from manufacturing plants in China. There are a few 100% UK made activated carbons made from 100% sustainable resources.

  • What is it the carbon made from? Coal (bituminous), oil, wood (hard, soft, sustainably grown?), coconut shell, other organic matter (all the way down to straw and waste).   

  • Can you use Biochar? - Some biochar carbons can act as both chemical filters and biological filters. They host “clean up” bacteria/microbes. It is the use of activated carbon as a biofilters that really excites SoilFixer. We believe our wood grade carbon will be a better chemical filter and act as a biofilter. As such it will last significantly longer without loss of filtering ability. We have evidence for this from use in filtering compost odours, but not in aquarium use.

Would you like to help us test?

If you have the kit to test and would like to help us formally test – please contact [email protected].     

Some of our advice may go against what other sources say. Clearly, we're not aquarium experts. All we ask when reading is that you take time to explore the alternative reasoning and try to match the experience you have to the type of carbon used and the causes and effects we list for different carbons. This will help clarify why there are so many different opinions.

There are scientists out there who can add layers of detailed chemistry. We have used broad descriptions that should aid the non-scientific person get to grips.

For what’s it’s worth, we have found one site that appears to have done a good job of testing and demonstrating evidence for different carbons in aquariums: Reef BSTV, a USA supplier of aquarium kits and author of the 52 part video series on you tube; see specifically the session on on pristine water.

What are you trying to achieve?

  • Crystal clear water. You spend lots of money on the fish, corals and lighting so you can watch them, a yellow tinge in the water spoils the view. Carbon removes yellow tinges, so use a carbon filter.

  • Non-smelly water. Water can start to smell, carbon removes odours, so add carbon.

  • Remove toxins? Carbon can remove some fish toxins.

A bit of background; well, okay a lot!

What are activated carbons, how are they made and classified?

Activated carbon is made by heating organic matter (coal, oil, wood, coco shells, straw, etc) in the absence of oxygen (this process is called pyrolysis or sometimes carbonisation). Organic matter (i.e. stuff that was once living plant or animal) contains carbon, hydrogen, oxygen, and a whole range of trace minerals (N,K,P, Mg, Mn, Ca, Fe). When it is carbonised, we are left with +98% carbon. This carbon has many forms – it can be a powder (amorphous, carbon black, soot),), graphite (sheets in various forms).

How many types of activated carbon are there?

There are hundreds of different grades. We estimate only four to five types are used in aquariums. You can classify activated carbons in many ways:

Types based on carbon source 

  • Bituminous carbon (i.e. derived from coal)
  • Oil
  • Coco shell
  • Wood

Types based on physical shape (powder, pellet, granule)

  • Pellets are made from carbon black (soot particles, powder) compressed with a binder into a hard pellet. 
  • Granules retain the original shape of the source material – most often they are wood or coco shells. 
  • Powder is the most common activated carbon made and sold. It is not sold or used in aquariums – you end up with black water!

Types based on pore sizes

Carbon pores can be thought of as small, medium or large – but remember we are talking atomic scale. Each pore size is better at trapping (adsorbing) certain molecules: small pores trap small molecules; large traps large. 
•    Micro pores (<2 nm) - amorphous powder has only micro pores. It has a high surface area and traps small molecules very well. Carbons that are steam of phosphoric acid “activated” will only be amorphous with only micro sites. 
•    Meso pores (2-50 nm) are typically found in coco shell carbon and wood carbon. Your medium molecules are phenols etc) 
•    Macro pores (>50 nm) trap large organic molecules. Aquariums can have lots of large organic molecules (think molecules of protein and tannins).  
•    Most carbons tend to have mainly one type of pore. Wood (when correctly processed) contains a mix of micro, meso and macro sites. (see why this is important below).

Types based on surface chemical structure
The activated carbon surface chemistry changes as the amount of hydrogen and oxygen is removed from the original organic source material. You can have carbons that are good at holding onto just negative charged molecules, or carbons that have negative and positive holding sites. This makes a big difference to what gets filtered. Please read further the detailed questions on what gets removed and what doesn't.  

Special grades tweaked to filter things that are not usually filtered
Carbon can be adjusted to filter many molecules that ‘standard’ activated carbon simply does not filter. It can be formulated to trap hydrogen sulphide, ammonia even mercury.
The big carbon manufactures each make >100,000 tonnes per year. They often list hundreds of grades of “activated carbon”. They make this many because carbon can be tailored to meet very specific industrial needs. Only a few are suitable for aquarium use. 

We can now look at a few of the aquarium filtration topics in a bit more detail.

Why are there lots of website contradictions on what activated carbon can filter?

One reason for this is (as stated above) is that carbon can be tweaked to absorb many different things. The problem seems to be that detailed specific information and general information get transcribed. Not all activated carbon is the same – even stuff made from the same source (e.g. wood) can vary enormously.

What causes the yellow tinge in aquarium water?

Most sites pin the blame on phenols and tannins. We will broaden this out using our knowledge of the decomposition (composting) process.

A yellow tinge in your aquarium water can be created by a myriad of organic compounds. They are termed dissolved organic matter (DOM) is used by chemists/sewage/water treatment sector. Almost all of them come from the decomposition of organic matter such as excess food, fish poo, dying plant and fish matter.

All this organic matter goes through the same biological decay process - large complex molecules (cellulose, lignin, DNA) are converted back in steps (through tannins, phenols, proteins, sugars and hundreds of other chemicals) down to amino acids and sugars and finally small molecules (various nitrogen ions, phosphorus, potassium, Ca, Mg, Na, CO2 and H2O). Decay is the reverse process of creating organic matter. A huge number of these natural decay products give a yellow (or brown tinge to water!)

All grades of standard activated carbon will remove the yellow tinge. Some will be better than others and this comes down to the ratio of small, medium and large molecules in the water causing the issues and the ratio of pore sizes in your activated carbon. We believe a mix of pore sizes is best. Some evidence supports this. (see reef test results)

Is my carbon Activated or not? Is SoilFixer biochar/Activated charcoal activated?

The definition for activated carbon is any carbon designed to filter – the definition covers all grades. Aquarium users pretty much only search for “activated carbon for aquarium use”. Suppliers not using ‘activated’ in the title simply won’t get found on eBay or Amazon.

However, certain grades of carbon have a defined “activation step” in the manufacturing process. The basic filter grade carbon is heated at very high temperature in the presence of steam or phosphoric acid. This “explodes” the bits of carbon from sheets stacked in layers into an amorphous powder (see photo on left and compare to wood on right)

Amorphous carbon powder

The 'activation step' hugely increase the surface area (from 400-900 m2/g to 1000-2000 m2/g). This all sounds great but you can’t use powder in aquariums as you'll end up with black water and you will probably get black over every surface in the house (ever tried changing an old-fashioned photocopier cartridge?).  

The powder is compressed with binders into pellets to make it easy to handle. This mixing and compression changes the filtering ability. (If you buy based on surface area number, take care to ensure the surface area is that of the pellet not the powder used to make the pellet!). Steam activated carbon pellets are exceptional at removing small gaseous molecules from gas streams. 

In liquid filtering (i.e. water in aquarium), your main target is to remove dissolved organic molecules (the yellow tinge). These are not small molecules on at the atomic scale! – they are medium and large.

Is high surface area good? Should you buy based on high surface area?

It depends on what you want to filter out - see activation above.

For aquariums, you want a carbon that filters the chemicals in your tank that cause problems. 

What are the different surface area tests and measurements for activated carbon

There are many ways to measure surface area. By far the most common (because it is quick, easy and highly replicable) is the iodine number. The iodine number gives a good approximation for the total surface area. The higher Iodine numbers the higher the surface area. Activated carbon have iodine numbers ranging from 400 to 2,000 mg/g of carbon and for our purposes we can directly relate these to the carbon surface which range from 500 – 2000 m2/g).  

Good carbon sources will quote an iodine number.  But this is not a full story for aquarium use! (If you have read the whole article we are repeating ourselves...). High surface area is fantastic at filtering small molecules, but not so great at filtering medium and large molecules. In your tank the yellow tinges and “tank” smell are caused by medium and large organic molecules. These are not filtered by micro pores which are directly related to high surface area. They are filtered by meso and macro pores. Activated carbons with this structure have lower surface area. (Having a lower surface area should not be confused with rubbish carbon that has low surface area because the pellets are crammed with filler clays and powders that block the surface area!)

The methylene blue number measure how good the carbon is at filtering the large methylene blue molecule. The molasses number is a broad indicator for how well large molecules are removed and hence indirectly carbons having large pores. Pellets tend to have poor methylene blue and molasses numbers but very high iodine numbers. Good Wood activated charcoal that retains the original structure will have a mix of pores and remove all three pores, but it will underperform in terms of the iodine number compared to a good pellet. 

From our composting odour filters we know wood is exceptional filter for natural volatile organic compounds products.

We seek to prove the same is true in water for the removal of dissolved organic molecules in aquariums.

Hands up – we launched our activated carbon for aquarium use in February 2018 without specific testing on aquarium water. We know it chemically pure and meets the BBF-QM standard and we know it filters gaseous VOC odours. Having done a lot of work on use of activated carbons in aquariums, we are committed to get comparative tests done on filtering aquarium water asap. Want to help – call us.

Why do some sites say activated carbon it will remove ammonia and others say no?

Ammonia (NH3) is a gas. As soon as ammonia gas passes out over the fish gills into water it instantaneously dissolves to become the ammonium ion (NH4+). If ammonia gas existed in tank water, it would bubble out of the water and no-one would have “ammonia” problems!

(We note that reef/sea water fish excrete excess nitrogen as urea in poo rather than ammonia via the gills – an adaptation to get around the fact that ammonia does not diffuse well into salt water).

This detail is relevant. If you ask a carbon supplier does activated carbon pellet remove ammonia – the answer will be no. (They know trying to remove ammonia (gas) from a gas stream passing over carbon pellets does not work. (They might say - choose type abc - it is a tweaked grade of carbon!)

For aquarium users, we should ask can activated carbon remove the ammonium ions from water?

For most suppliers of pellet, the answer will still be no – the surface properties of amorphous powder are not great at trapping the positive ammonium ions. But, for granular chip made from wood – the answer can be YES. In our filtering experience, wood charcoal removes both ammonia and ammonium very well.

(If you dig deep into the academic papers, then you’ll find explanations related to a number of chemical and surface changes depending on temperature and the original chemical structure. At very high processing temperatures (>800 C) you make amorphous powder which is devoid of any negative charge sites that would attract a positive cation. In lower temperature processing, the porous wood structure (see photo) and some anionic sites are retained. These sites trap negative ions. (The science would say low temperature biochars have a low CEC and relatively low AEC. High temperature chars have moderate CEC and no AEC (see photo).

SoilFixer Activated Carbon - Wood Charcoal pores

If this is not already complex enough; the story takes a twist when you look at certain wood types.

Coco (processed at moderate temps) will retain the original medium pores. Hard woods processed correctly will retain a mix of small, medium and large pores). Soft woods normally “explode” and lose the pores leaving an amorphous powder with no medium or large pores.  (micro pores <2nm, meso 2-50 nm, macro >50 nm).

Bacteria and fungi are tiny (5-100 nm) and can fit into macro and some meso pores, but not micro pores. They cannot live in micro pores. Some wood biochars are therefore excellent homes for microbes. Microbes consume all forms organic matter and nitrogen derived from organic matter decay. There are generalist microbes who consume ammonia and ammonium and specialists who consume either nitrate or nitrite.

Just like us, they use the nitrogen to make amino acids and in turn proteins/DNA.

In your aquarium, having a population of “clear up” bacteria that consume organic matter and nitrogen would be a bonus. Even if this population is held outside the main tank in a biofilter (see below) canister, refugium or sump.

The SoilFixer biochar activated carbon routinely removes ammonia/ammonium from compost odours. We believe the same will be true in aquariums.

How long will the carbon last? How often will I need to change it?

Frustratingly for the user, all manufactures say the same – it depends. And it does; on how much leftover food (organic matter) there is, how many fish, how often the water is exchanged, the carbon used, and so on. 

Chemical filtering knowledge informs us that once the carbon sites are filled with trapped molecules, the carbon needs replacing.  We’d like to take a detour here and ask you to consider an alternative way SOME activated carbons could be used in aquariums!

We know, our activated carbon has been used to remove compost odours (including ammonia/ammonium) and it is still actively working after five years.  Yes, FIVE years.

We believe this is because our wood-based biochar (activated carbon) hosts microbes. These microbes do not just live on the surface (per pellets) they inhabit the large (macro) pores in the biochar. The pore size is critical only carefully made wood-based charcoal retain a mix of small medium and large.

Once living in the pores, the microbes “eat” the trapped organic matter releasing the site to trap another molecule from the water. You have millions/billions more bacteria continuously cleaning the water in a self-sustaining filtration system.

When we thought about this - no one changes the filtration media in a natural pristine reef system – the ecology sorts it out.

Does the carbon clog up with bacterial film (slim) and stop working? 

The microbes will generate lots of biofilm (the slimy stuff), but it should not clog up. There is a bit of work to balance the flow rate and volume of carbon, but you should within weeks reach ecological balance where the microbe population grows and declines as the amount of waste comes and goes.  

In theory, an activated carbon biofilter should only need an occasional flush with water to remove excess biofilm (the slimy stuff). Don’t remove all the biofilm. Once flushed, it should be good to use again and again.

(This is totally different to using activated carbon as chemical filter. In chemical filtration, once the surface sites are full of trapped molecules it is spent and needs to be replaced).

If biofiltration works, how come it is not used industrially for drinking water?

In drinking water filtration, the gaps around the activated carbon pellets clog up with bacterial biofilms. There is always a risk that microbial films host bad bacteria as well as good “clean up” bacteria. In human drinking water the risk of bad plus the need to remove biofilms from the system means biofiltration is rarely used in human drinking water - sterilisation to kill bacteria is the used.  In aquariums you cannot sterilise the water! 

It might not be good for repeat sales of activated carbon – but biofilters offer a potential leap forward in the way activated carbon is used in aquariums. Biofiltration could take place in the filter canister, but more promising would be setting it up in a sump of refugium. We look forward to proving the theory with any aquarium users who have the test facilities to measure and report he results.

Are the nitrogen bacteria the same as other bacteria?

No.  There are four main ways microbes consume nitrogen. Most bacteria (the decomposers, clean-up team) consume ammonium and/or organic compounds containing nitrogen (proteins, amino acids, urea, uric acid). Hence, they will clear up nitrogen entering the tank from fish gills and/or excess food. Here’s the rub – when there is more nitrogen-based food than clean-up bacteria can cope with, the excess ammonium builds up in the tank. (In all living cells - consuming nitrogen is essential to life, but excess nitrogen is toxic to cells, so excess has to be removed) 

The Nitrifying bacteria (Nitrosomonas, Nitrosococcus, Nitrobacter) are specialists. They use (and hence remove) nitrate or nitrite ions dissolved in water. If you have more ammonium than can be consumed by the clean-up microbes, a population of nitrifying microbes is likely to develop.

The nitrifying should not be confused with the other specialists: the de-nitrifying bacteria. They operate in the opposite direction. They only work in low oxygen (anaerobic) conditions. Your fish will be dead before oxygen levels are right for these microbes.

Finally, the nitrogen fixation microbes – they convert nitrogen gas (in the air) into nitrates/ammonium. 

The Nitrogen cycle is much more complex than the carbon cycle – two simplified version can be seen below: 

Nitrogen cycle in Soil

 Will wood charcoal remove nitrate (NO3-) and nitrite (NO2-)?

Most activated carbons are poor at trapping these negative ions. If bacteria grow on your carbon, you will get a net reduction in these ions. We have strong evidence wood biochar hosts bacteria and removes (via microbes) nitrogen compounds. We evidence pelleted carbon does not host and remove nitrogen as effectively – see biofiltration below.

Does activated carbon contain ash?

It depends on what you mean by ash. A lot of sites seem to refer to ash when it looks like they really mean “fine powder” attached to pellets or granule.

What is ash?  When you fully combust organic carbon in oxygen (i.e., burn it in air), you will be left with a white ash (think about your BBQ ash left in tray the next morning). This ash consists of the oxides that are non-combustible. It will be +60% calcium oxide, around 10% magnesium oxide, some iron oxide potassium and manganese. It will also have traces of most of the essential metal oxide required for life! You might also have silica depending on the plant source. You will have a different ash composition if you’re raw material for carbon production was coal (coal ash composition is impacted by the soil and rock surrounding the coal deposit.  Some activated carbons will measure ‘soluble ash’ content. 

When you manufacture activated carbon from wood, carbonising the raw material will convert some of the bound minerals exposed on the surface to oxides. However, creating lots of ash means you are losing carbon – i.e. wasting product. Good activated carbon will have little free surface ash that can move into the water. It will always retain some bound (non-dissolvable) oxides on the surface. Small amounts of ash are not problematic in tanks – the minerals making up ash are needed by fish and bacteria. Large amounts could be problematic as the ash is alkaline in nature (12) so it could affect pH. (Note pH of our activated carbon is 9.5). If you are flushing the activated carbon before use – this takes a minor issue and makes it miniscule issue.

Are there are other items that get confused and called ash

All carbon particles fragment into tiny specs. These specs are silver grey when dry and black when wet. Black dust in activated carbon is not toxic, but it is bad news because it creates a blue / grey / black water depending on how much there is. The black dust is usually removed by flushing in water. 

  • Granular carbon will have a high proportion of carbon powder
  • Pelleted carbon will have a low amount of powder.

You should always flush granular carbon before use to remove carbon powder.  This black powder is not ash in the sense that a carbon manufacture uses the term ash.  

Pelleted carbon is made by binding carbon powder. The binders are usually simple sticky materials like starch solutions. However, beware - if you look at pellets they tend to vary in colour from light grey to black. Assuming they are all super dry (colour also changes with moisture / dryness), then the grey pellets are more likely to have an inorganic clay type filler added to bulk out the binder. These filler powders are, to all intents, “ash” oxides. (You can closely relate this practice to your BBQ briquettes, some are made from pure carbon and burn well, others add clay/oxide fillers to the briquette (cheaper) but they do not burn as well.

Do some activated carbons add phosphorus to the water and create havoc?

Phosphoric acid is used in the manufacture of some “activated carbons”. We have not had the time to trace all the links to know if this leaves a residue of dissolvable phosphorus compounds.

Most activated carbon do not use this process step, however, all activated carbons made from wood and plant organic matter will contain phosphorus because it’s key to life/cell structure. Some of this will be converted to PO4 on the surface of the activated carbon. Little of this will be soluble in water.  All organic matter contains phosphorus, so excess food will also add PO4 to the tank! Excess PO4 can cause algal blooms etc.  

Does activated carbons add Fe2O3 (iron oxide) to the tank

This is very unlikely. See the section on ash. The amount of soluble iron oxides on the activated carbon surface is very small. Even when all the carbon is fully combusted to ash, the iron content in wood charcoal ash tend to be in the sub 100 ppm (i.e. mg/g). We have read some sites tracing iron oxide issues back to ferrous oxide filters used to remove phosphorus. We have not had time to look into this.

Can I regenerate my activated carbon?

There are some claims online that you can regenerate activated carbon by heating it in a domestic oven. This seems to be a ‘technology transfer jump’ from the industrial process to re-activate carbon. Once fully used, activated carbon will contain lots of organic molecules. Industrially, this spent carbon can be reactivated by removing the trapped organic molecules by re-heating the carbon to 800°C in inert atmosphere. (i.e. repeat the original manufacturing process).  This is only possible in controlled situations where you know what organics have been trapped and hence know you are not going to release any toxic products. Regeneration is costly and not easy even for the big companies.

 In a domestic oven, you will get to maybe 300°C – nowhere near hot enough to carbonise. You might combust (burn in oxygen) some of the trapped organics. They will volatilise off and be released into the kitchen air. This limited removal in view does not gain you much.  

Note: if you can achieve higher temperatures - without an inert gas or specialist pyrolysis system, you will combust the carbon (that is, lose it!). Burning organic matter can release toxic gases (e.g. PAHs) and some organics can convert to further toxins (e.g. dioxins). One reason why the EPA controls burning of all wastes.

Can I extend the life of activated carbon?

Bacteria will consume organic matter trapped in activated carbon. In theory running the carbon filter in a biofilter mode and occasionally removing excess biofilm (slim) by flushing with water will significantly extend use.

However as with any microbial system, you must take a decision around the risk factor related to good (beneficial) bacteria and bad (pathogenic) bacteria – be they bad to humans or fish. Most bacteria are not harmful to fish or humans; many are beneficial and a tiny few are pathogenic. Normally the good bacteria out compete the bad – this is true is compost heaps and river bed muds. But there is always a risk that the bad guys take over. The way to completely remove the risk is sterilisation - i.e. kill 100% of all bacteria (good and bad). This is the approach taken in human drinking water and kitchen cooking surfaces. As we are finding out from extensive use of antibacterial chemicals, this brings a new set of problems:  many of the good guys are actually essential to human health – they help build our natural resistance ready to defend against the bad guys. The bad guys adapt new defences and become super bad (e.g. MRSA).

We are keen to properly test activated carbon (our biochar) as a biofilter in sumps and or refugiums. Contact us if you are set up to test.

Will activated carbon remove chlorine from water?

Yes activated carbon chemically reacts with the chlorine (or hypochlorite ion) in water to remove it by converting the chlorine to Hydrogen Chloride (HCL). Hence some of the carbon granule is used up. In human water consumption, HCL (at low levels) is non-toxic and has no ‘taste’. Hence polishing the water through activated carbon removes the taste of chlorine.

Activated carbon is used on a huge scale to dechlorinate drinking water. A reason chlorine is added to water is to kill bacteria (sterilise it).

A couple of things to note:

  • Activated carbon is not removing the chlorine molecule via adsorption.
  • Hydrogen Chloride (HCL) is acidic, so you neither want a lot of HCL or chlorine in the aquarium water.
  • Chlorine in tank water can be resolved by other methods such as ‘ion exchange’ filters.

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