Carbon dosing in saltwater aquariums | How to lower nutrient levels with liquid carbon sources and biopellets, and precautions to note

"Carbon dosing" becomes an option when water changes, protein skimmers, feed adjustments, and nutrient export via refugium alone are insufficient to lower nitrate and phosphate levels, or when you want to more actively target low nutrient conditions. There are methods using liquid carbon sources or biopellets, but starting without understanding the mechanism can lead to oxygen depletion or bacterial blooms in the tank, so it is necessary to understand the mechanics and precautions before introducing it.

How nutrient levels decrease

Nitrate and phosphate have limited removal methods other than water changes when they remain dissolved in water. Carbon dosing is an indirect method that removes these dissolved inorganic nutrients by converting them into a "recoverable form" in the bodies of organisms.

Heterotrophic bacteria, which proliferate using organic carbon as a nutrient source, are always present in the tank. When additional organic carbon sources such as ethanol, acetic acid, or sugars are provided, these bacteria proliferate while taking up nitrate and phosphate from the water along with the carbon source. The increased bacteria are captured in the bubbles of the protein skimmer and removed from the tank, or become food for other livestock, ultimately transporting nutrients out of the system. In other words, carbon dosing itself does not decompose and eliminate nutrients; rather, it is a technique based on the concept of "converting dissolved nutrients into bacterial biomass that can be removed."

Liquid carbon sources and biopellets

There are two main approaches to supplying carbon sources.

• Liquid carbon sources: A method of adding a liquid of ethanol, acetic acid, or similar diluted in water at a fixed amount each day. The reaction to see results is relatively fast and dosing adjustment is easy, but it is prone to inconsistencies such as forgetting to dose or adding too much.
• Biopellets: A method of supplying carbon continuously by placing biodegradable plastic pellets in a dedicated reactor where they are exposed to gentle water flow, causing the surface to gradually dissolve. The risk of forgetting to dose is small, but management of the equipment such as adjusting the reactor flow rate and checking pellet consumption is necessary.

With either method, adequate skimmer capacity is essential to properly remove the increased bacteria from the tank. If you increase carbon sources while skimming is weak, the bacteria remain in the water column, which can degrade water quality contrary to your goal.

Basic principles for getting started

With carbon dosing, the basic approach is not to introduce the target amount immediately, but to start with a small amount and increase gradually over several days to a week while monitoring. Observe nutrient measurements alongside skimmer foam production and water color changes; once nutrient levels begin to drop, slow the pace of increases. Livestock, especially SPS corals, are sensitive to rapid nutrient changes, so rather than trying to jump straight to low nutrient conditions, careful observation of numerical trends and stepwise adjustments are essential.

Risks of overdosing

The most important concern with carbon dosing is the risk that arises when the balance between dosing amount and bacterial growth rate is disrupted.

If bacteria increase too rapidly, dissolved oxygen in the water can be heavily consumed by bacterial respiration during times when photosynthesis does not occur, such as at night, potentially causing oxygen depletion. If you observe fish breathing heavily near the water surface or coral polyps closing, these are signs of oxygen depletion to watch for. Additionally, the rapid increase in bacteria can cause temporary clouding of the water—a "bacterial bloom"—or an imbalance in nitrogen and phosphorus uptake can make cyanobacteria more prone to growth. These are all problems that commonly occur as a result of "increasing the amount too suddenly," and the most certain way to avoid these risks is to follow the basics: starting with a small amount, increasing gradually in stages, and ensuring adequate skimming capacity.

Risks of lowering too far

Attention must also be paid to the risk of carbon dosing being "too effective." When nutrient levels are reduced to near-zero concentrations, the zooxanthellae that are symbiotic with the coral may not get enough of the nitrogen and phosphorus they need, causing zooxanthellae density to decrease and the coral's color to appear faded or washed out. This is not disease but rather a result of overzealous nutrient management in many cases. Even if the numbers show beautifully low nutrient levels, also observe whether the coral's color and tissue condition have changed, and be prepared to reduce the dosing amount by one step if necessary.

Guidelines for starting and stopping

Carbon dosing should not be thought of as something "once you start, you continue forever"; rather, it is easier to manage if you treat it as something that is adjusted based on circumstances—increasing it temporarily during periods when nutrient levels are higher than expected, or reducing the dosing amount if conditions improve. Stopping suddenly can cause nutrient levels to rebound as they are no longer being suppressed by bacterial growth, so when stopping, as with increasing, it is safer to reduce gradually while monitoring the numbers. Taking a baseline measurement of nitrate and phosphate before starting, and then continuously recording them at the same time with the same measurement method after beginning, makes it easier to determine when to increase or decrease.

Approaches to nutrient management in saltwater aquariums covers nutrient management methods other than carbon dosing, such as water changes, feeding, and refugium use, so consider this along with finding the combination that works for your tank. For more on the relationship between nutrient levels and coloration, also refer to An introduction to color enhancement in SPS corals.