Aquarium Decorations     Activated Carbon     Water Chemistry     Aquarium Lighting     Snails     Quarantine Tank     Cloudy Water     Aquarium Gravel    Aquarium Safety

Activated Carbon in the Aquarium

Just what is activated carbon?    What is the best kind to buy?   How long does it last? Should I use it in my aquarium?   These are some of the questions I will attempt to answer.

Activated carbon is carbon that has been treated with oxygen in order to open up millions of tiny pores between the carbon atoms.  It can have anywhere from 300 to 2,000 square meters of surface area per gram, or 116 to 750 square feet per ounce.  Activated carbon absorbs various substances, from gases and liquids.  By absorb, I mean attaching to the pores by chemical reaction.  When certain chemicals pass next to the carbon surface, they attach to that surface and are trapped. 

Carbon can be made from animal, mineral, or plant-based materials such as bituminous coal, lignite coal, various woods, coconut shells, peat, and animal bones.  The types of activated carbon include granular, pelletized, and powdered.  It can be acid washed or non-acid washed.

 Activated carbons are also used in liquid, gas, and air filtering.  Many of the home water-filtering systems use activated carbon.  It is an effective means of removing dissolved organic compounds (DOC's) from the aquarium.   Activated carbons made from bituminous coal are the most effective for aquarium use.  They contain a wide range of pore sizes, enabling it to remove a wide range of organic pollutants.  Coconut-shell carbon, on the other hand, has a large surface area but very small pores and many of the organic compounds are too large to fit.  This type of carbon is primarily designed for gas filtering.  Lignite coal produces carbons with the opposite problem.  Their pores are large enough for common organic molecules but has a reduced total surface area.  It can be used, but you will need a lot more carbon to remove the same amount of organic material than carbon made from bituminous coal.

 Do not buy a carbon just based on surface area claims.  Read the label and determine the type of base material.  If the package doesn't state the base material, e-mail or call the manufacturer.

Here are some general guidelines I picked up from somewhere, (can't remember where exactly to give him credit) that can help you select the best carbon for the job.

If the label gives key specifications (porosity, density, ash, and phosphate content) then the supplier likely has nothing to hide and the carbon is likely a good one. If the carbon boasts no phosphate, then the supplier is either lying or doesn't know any better.  Neither is very reassuring. Compare weight and volume.  The less weight for a given volume, the greater the porosity and the better the carbon, all-else being equal.  You can usually do this without buying the product first.  After buying the product, this parameter will be reflected by the carbon's ability to float and fizz. Some physical characteristics that should be evident before buying should be considered.  Particle size should be about pinhead.  Powdered carbons offer no real advantage and are difficult to handle.  Large particle sizes become impenetrable by water and so only the outer 1-2 mm of the particles are absorbent, making as little as 20% of the carbon useful.  Spherical shape is ideal hydro-dynamically for unimpeded water flow and inability to pack, and is therefore superior to random granular shapes.

The use of activated carbon can increase phosphate levels in the aquarium.  These increased levels can lead to algae growth.  All carbons are organic in source, therefore rich in phosphates.  Acid-washed carbons will leach less phosphate than others and are usually more expensive, since they have had much of their ash and phosphate washed out.  Ash is important because it is responsible for "pH shock".  Some carbons can increase pH to over 10 in a very short time.  An acid-washed carbon will barely increase pH to 7 over several days.  Carbons that do not alter pH are usually the same carbons that will not leach much phosphate.

All carbons should be rinsed well before being placed in the filter system.  A yellowish tint in your water can indicate the need for carbon replacement.  It can lose a large percentage of its effectiveness in two to four weeks.  Organic materials and bacterial slime coat the surface blocking access to the inner pores.  Rinsing the carbon between changes can extend its useful life.

As a general rule of thumb, it is better to use less carbon and change it more often.  The amount I hear most often is roughly 100 ml per 30 gallons with monthly changes.  Another general rule is, the lighter the weight of the carbon for a given volume, the better.

When treating fish with medications, the carbon should be removed from the filter.  After treatment, the carbon can be put back in to help remove the medications although water changes should be performed in addition.  I would recommend changing the carbon a week to two weeks after you have used it to remove medications.

The use of activated carbon with live plants is greatly debated.  Generally, planted-tank aquarists do not use activated carbon for fear of losing trace elements needed by the plants.

The following can help give you an idea of what compounds can be removed by the use of activated carbon.

Absorption Potential of Various Substances by Activated Carbon High to very good    Arsenic, bleach, chloramine, chlorine, chromium, colors, dyes, gold, insecticide, odors, monochloramine, tin Good to Moderate    Acetic acid, cobalt, detergent, hydrogen sulfide, mercury, ozone, potassium, silver, soap, solvents, vinegar Fair    Copper, iron (not chelated), lead nickel, titanium, vanadium Low to None    Alkalinity, ammonia, barium, carbon dioxide, hardness, copper, manganese, nitrates, selenium, molybdenum, zinc.