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What Is Carbon Dosing and How Does It Help Your Reef Tank?

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Controlling nitrates is one of the most common problems on keeping a saltwater tank. Carbon dosing may handle excessive nitrates and phosphates; however, it takes some knowledge to perform correctly. Today’s article is an introduction to carbon dosing. 

Here is what you’ll learn:

  • What is Carbon dosing
  • How does Carbon dosing help
  • When Carbon Dosing is Helpful
  • Safe Carbon Dosing Application (specific dosage)

Without further ado, let’s get started

What is Carbon Dosing?

Carbon dosing is one of the ways to fight back elevated nitrates and phosphates.

The concept first came from large-scale commercial aquaculture. Let’s say X farm wants to grow around half a million fish in a round tank. Naturally, shareholders want these fish to grow as fast as possible. One way to do that is overfeeding. Eventually, the tanks lock up with high nitrates, phosphates, etc. 

A new problem raised interest in carbon dosing. Basically, a proper dosage of carbon promotes the growth of nitrates-feeding bacteria. That is why carbon dosing is best known for lowering nitrates and phosphates.

There are lots of ways to carbon dose. Here are some of them:

  • Vodka dosing
  • Vinegar 
  • Granulated sugar
  • Specifically-made products 

Even though the purpose behind these methods is the same, you can not use them interchangeably. If the phosphate level is too low and you do aggressive carbon dosing, you start to feed the bacteria and algae.

It’s crucial to measure your phosphates beforehand. For instance, the last thing you want to do is put a bunch of vodka, vinegar, or sugar if your phosphates hold 0.2 or even lower. 

How Does Carbon Dosing Help?

Carbon dosing gives the coral polyps a way to get phosphates. It’s particularly important because most reef-keepers leverage tanks with lowered phosphates. 

The coral polyps have an excellent mechanism for taking nitrates(NO3) out of the water. The problem is corals don’t really need NO3. Corals can live without nitrates but can not tolerate zero phosphates (PO4) levels. The exact opposite situation occurs with bacterias (those are known for eating elements); they soak up phosphates while being mediocre NO3 digesters. 

Why should you even care about nitrates?

Nitrates are the last step in the nitrogen cycle. They turn toxic fish – and other – wastes from ammonia into nitrites. Then nitrites are transformed into the least toxic form known as nitrates. Even though nitrates per se are not dangerous, they still can be extremely harmful to fish and corals. Even moderate levels can cause corals to turn brown and die. Nitrates can also be responsible for fish deaths. Shortly, elevated nitrate level — bad news for your display tank. 

There are two most common situations when nitrates can outburst. 

A young tank that doesn’t have the biological filtration to support fish in it. You can either media support or enjoy a clogged, half-dead reef tank. Make sure to check out my article about one of the most efficient media — ferric oxide, a.k.a. GFO. It’s a specific form of iron to inject into the tank. 

Poor display tank husbandry. That would be:

  • Overcrowded aquarium. Too many fish produce a lot of wastes; thus, toxins accumulate.
  • A tank that doesn’t have enough rocks. Rocks are known for keeping the ecosystem balanced.
  • An overfed tank. Again, excessive amounts of waste that pile up over time.
  • Little or no water changes at all: partial changes keep the aquarium healthy. 

Even though managing nitrate levels can be both reactive and proactive activity, you may want to stay as dynamic as possible during this battle. However, sometimes you have to stay apart: add some organics and let nature do the job. For instance, you’ve tested parameters within the tank, and you noticed that nitrates are way too high. Of course, you can perform 50% water changes twice a week; but, that would be reactive (in other words, inefficient). What you can do instead is carbon dosing or any media filtering. 

When Carbon Dosing is Helpful

Here are the 4 main reasons to leverage carbon dosing:

  • Minimizing nitrate levels
  • Decrease in phosphate
  • Limiting cyanobacteria growth
  • Decrease in dissolved and suspended organic matter 

Minimizing Nitrate Levels

Optimal carbon dosing helps to counter elevated nitrate levels. The effect is achieved by increasing the number of bacteria which absorb the excess nitrates and then remove them from the aquarium by filtration (usually the role of the filter is performed by a protein skimmer). In addition to their direct effect on nitrates, bacteria are also an excellent food source for organisms that feed on nitrates. Sea sponges, for example, convert nitrates into compounds that are more beneficial to the aquarium’s flora and fauna. 

The dosed carbon also helps bacteria that are in areas with low oxygen levels. Such bacteria use nitrates instead of oxygen. Nitrogen (N2) is produced in the process. The remaining bacteria can directly convert nitrates into nitrogen, which then leaves the aquarium naturally. The gas simply volatilizes leaving no trace of nitrates. Carbon dosing promotes the absorption processes, which leads to more nitrate absorption. It is worth noting that the more rocks and sandy substrate you have in the aquarium, the more effective carbon dosing will be. Depending on the type of carbon dosed, (vodka, vinegar, sugar, and so on) permeability may differ, but they all stimulate nitrate absorption processes.

Decrease In Phosphate

Injecting carbon (regardless of its type) into the aquarium reduces phosphate levels. The process is much less intense compared to the reduction of nitrates. On the one hand, this is because bacteria use about 16 times more nitrates than phosphates. The ratio can vary greatly, but phosphate is always absorbed much more slowly. This difference is due to the chemical composition of substances present in living cells. On the other hand, the process of nitrate absorption with the release of nitrogen is another reason why phosphates are absorbed more slowly than nitrates.

Bacteria tend to accumulate phosphate within themselves while reducing the phosphate content of the water. Corals will then eat these bacteria and there will be very little change in phosphate levels, however, this process has a positive impact on the health of the corals. 

Therefore, if you have severely elevated phosphate levels and low nitrate levels, GFO is your best bet. Be sure to read up on what it is and how it can benefit you. Of course, you can use carbon dosing, such as vinegar. However, this method is best used in the short term; to maintain phosphate and nitrate levels. Many reef keepers combine carbon dosing and GFO. 

Limiting Cyanobacteria Growth

A scientific study found that the population of cyanobacteria can decrease, increase or not change at all depending on the type of carbon dosing. For example, vinegar does not affect the number of cyanobacteria, while vodka directly increases their population in the aquarium. 

Cyanobacteria produce special substances for energy conservation in emergencies. These substances are part of some biopellets. In other words, the bacteria can use biopellets. Most reef keepers claim that bacteria-related problems occur less often if carbon is dosed.

This is scientific research I relied on

Decrease In Dissolved And Suspended Organic Matter 

Bacteria recycle substances undesirable to simpler organisms. For example, suspended solids or substances dissolved in water. Although the bacteria produce toxins in the process, this is a minor drawback. Most reefers use activated carbon, which easily neutralizes the harmful substances formed during processing.

Safe Carbon Dosing Application (Specific Dosage)

Once you decide to perform carbon dosing, one of the easiest ways is vinegar. Do not stick to numbers from this table unless you’ve measured phosphates. Some carbon dosing methods are dangerous for tanks if phosphates are too low (lower than 0.2). 

MEASURE YOUR PHOSPHATES FIRST! If they are higher than 0.2, you are safe to try vinegar dosing. If not, consider special and more gentle products. For instance, the Tropical Marine brand is known for a wide range of anti-nitrate products.

Vinegar (5%) ml.
Reef Tank Volume (gallons)
25 Gal.50 Gal.100 Gal.150 Gal.200 Gal.
Week 1 (Day 1 to 3)
Week 1 (Day 4 to 7)
Week 39.611.214.417.620.8
Week 413.615.218.421.624.8
Week 517.619.222.425.628.8
Week 621.623.226.429.632.8
Week 725.627.230.433.636.8
Week 829.631.234.437.640.8
Week 933.635.238.441.644.8
Week 1037.639.242.445.648.8
Week 1141.643.246.449.652.8
Week 1245.647.250.453.656.8
Week 1349.651.254.457.660.8
Week 1453.655.258.461.664.8
Week 1557.659.262.465.668.8
Week 1661.663.266.469.672.8

Generally, carbon dosing is a great way to get rid of phosphates and nitrates. It also helps to maintain your corals healthy in a low-phosphate environment. Bear in mind that not all methods of carbon dosing are the same: some of them are more aggressive than others. For instance, dosing ethanol (vodka) is one of the roughest ways to treat nitrates. Do not dose if your phosphates are too low.