Freshwater Aquariums: How to Dose a Plant Tank, Part 1: Carbon Dioxide

Clearly one of the main factors that attracts people to planted aquariums is the apparent ease of maintenance and how easily plants can grow under a wide range of conditions without any help from the aquarist. This is absolutely true but it is not always guaranteed that plants will take to your particular system and it is mostly applicable to the weedy type of plants. The aquatic gardener who chooses to never dose CO2 or any kind of macro or micro nutrients will be shooting themselves in the foot and missing out on a big part of the freshwater plant hobby. Without dosing it is much more difficult to grow some of the more exotic water plants and plants that are more robust, more beautiful and more colorful than those that are left to scavenge for what limited nutrients may be produced in the aquarium.


Steven Chong Wabi-Kusa Aquascaping.


Very much like some of the articles published earlier this decade which claimed that fish food alone was more than enough to satisfy the trace element needs of corals and reef life, there is a school of thought within aquatic gardening that is content to let the livestock population provide the bulk or all of the nutrients that aquatic plants receive. The Natural Planted aquarium style of aquatic gardening tends to be less occupied with how plants are arranged and without more than the bare minimum in lighting, substrate and dosing aquarium water. While there is something to be said about the elegance of growing a planted tank with little to no effort, if growing the most robust and lush looking plants possible is your aim then you will want to get to know the importance of macro and micronutrients for plant growth. The Macro Nutrients that are most important for day to day plant growth are Carbon Dioxide, Nitrogen, Potassium and Phosphorus (or CO2 and NPK for short). This article
will focus on the importance of carbon dioxide with the next installment dealing more with the use of macro and micro additives for the planted aquarium.


For growers of “air” plants, carbon dioxide as a nutrient is never an issue but in the aquatic environment, CO2 is almost always the most limiting nutrient to plant growth; without CO2 photosynthesis doesn’t even get going and your plant growth will reflect the extent to which you are adding carbon dioxide. Like using a protein skimmer in a marine aquarium, dosing carbon dioxide for aquatic plants one of the few keystone husbandry practices: with adequate CO2 your plants will grow better, faster, and they will be better able to outcompete undesirable algaes that can take over and cause so many people to walk away from aquatic gardening. Also, like a coral’s relationship between lighting intensity and water flow speed, the more light you give a water plant the more CO2 it will require for the photosynthetic reaction. If you place strong light on your planted aquarium without adding a balanced amount of carbon dioxide, you will be more likely to encounter algae problems.

There are many ways to provide and deliver carbon dioxide to a freshwater planted aquarium. The CO2 can come from a large cylinder, a small canister, seltzer tablets or from the fermentation of sugar by yeast. The DIY “yeast bomb”, as I like to call it, is the cheapest way to begin dosing carbon dioxide. The yeast bomb works well enough but depending on the mix the carbon dioxide production will only produce a reasonable amount of CO2 for 7-10 days with a highly variable output depending on the recipe, temperature etc. This type of entry into dosing carbon dioxide is ideal for tiny tanks, or aquariums which really have a low demand for CO2. Seltzer tablets are a step up from the yeast bomb and they work by producing all of the day’s CO2 at once and then usually storing that gas in a bell which is submerged in aquarium water. The CO2 gradually dissolves in the water but the seltzer tablet must be added on a daily basis. The most reliable way to dose CO2 is through the use
of pressurized cylinders and two stages of pressure regulation. The most typical setup uses your run of the mill CO2 gas cylinder and dispenses CO2 as controlled either by a manually adjusted needle valve, a pH controlled electronic solenoid or both. In the case of the pH controlled solenoid, the hardness of the water and the setting of the pH is used to keep the CO2 concentration at a steady level. As the plants use up carbon dioxide and the pH rises, the pH controller will open the solenoid and release carbon dioxide based on the same downward control that is used for calcium reactors. The miniature pressurized CO2 gas delivery system is becoming increasingly popular because it is reliable and affordable, so long as you don’t have to pay too much for the disposable CO2 cartridges or you don’t have to replace them too often.

The delivery method of carbon dioxide from a gas to dissolution into the aquarium water will also depend on the aquarium set up. The use of ceramic diffuser discs is by far one of the most popular methods for delivering CO2 as this method is great for moderate demands of carbon dioxide and smaller tanks. The ceramic diffuser have a very fine pore ceramic disc which “sprays” a fine mist of tiny CO2 bubbles that mostly diffuse into the water column before they reach the water surface. It is important to have the smallest bubbles so that they will slowly rise and have more time to dissolve into the water. CO2 diffusers are straightforward but over time they tend to clog up and require regular maintenance to make uniformly small bubbles. The exception is the inline diffuser by Cal Aqua Labs which is completely contained inside a vessel that is placed inline and downstream of a canister filter. A simlar yet more malleable method of CO2 delivery is the use of inline CO2 reactors. A co2
reactors is a simple cylinder that is also placed inline and downstream of a canister filter or other water stream. The widened cylinder of the CO2 reactor slows down the speed of water in the and a range of materials can be added to the cylinder to increase turbulence and the dissolution of CO2 into aquarium water. A CO2 reactor with a diameter of two inches and a height of ten inches can easily handle to dissolution of enough CO2 to satisfy the needs of a highly stocked 300 gallon aquarium. Some other ways to inject CO2 to planted aquaria involve the use of venturis or needle wheel pumps but these techniques are on the fringe of the mainstream and not widely used.


Carbon dioxide diffuser.


Carbon dioxide reactor.



A DIY carbon dioxide reactor made with a yeast/sugar solution.

The last several years have seen a wide range of new products coming to market which promise to deliver a soluble carbon source that can approximate using actual carbon dioxide. One product which has gained widespread usage is Seachem’s Flourish Excel product. Flourish Excel is more or less embalming fluid and when used in a planted aquarium it delivers a range of benefits beyond simply being a soluble carbon source. When first added to the aquarium, Flourish Excel acts as a mild algaecide with a molecule that binds the pores of single cell organisms and causes them to burst. Seachem doesnt advertise this algae killing property because it would then need to register the product as a controlled substance with the EPA and then we would all have a harder time getting the product. In addition to behaving as an algaecide, Flourish Excel is said to improve the uptake of iron and it breaks down into carbon dioxide after about seven hours in solution. I have found that adding Flourish
Excel is a practice which complements my CO2 dosing regimen and it also helps to grow plants that are more lush and free of algae.

As far as measuring CO2, there are a couple of techniques used to indirectly measure the level of CO2 in your aquarium. The relationship between pH, carbonate hardness and CO2 is most often used to determine how much CO2 is in the aquarium water. Small and medium sized drop checkers are one of the most effective ways to tell the amount of CO2 in your aquarium: these small inverted glass bubbles are filled with aquarium water and a few drops of a special pH indicator solution: the contained fluid will change color so that it will be blue when the CO2 is low, yellow when it is too high and green when the CO2 is just right. The drop checker is an indiscrete way to measure CO2 in two ways: First it does not convey a true pH concentration, only a relative concentration and it also can be obtrusive since it requires being placed inside the aquarium water. Another old-school CO2 measuring technique uses the movements of algae eating shrimp as a proxy: in this method the aquatic gardener
is advised to increase the rate of CO2 injection until the movements of the algae eating shrimp are noticeable slowed. If the CO2 gets too high the shrimp will become lethargic at which point you should reduce the CO2 injection until the shrimp returns to grazing at an intermediate speed. It’s kind of hokey I know but it works.


Drop Checker.

Whether your plants even require any or little dosing is entirely up to you. There are plenty of setups of slow growing ferns and mosses that are perfectly balanced and grow at a rapid pace on their own. For the majority of planted tanks however, a little addition of carbon dioxide will go a long way towards making your plants grow more lush and beautiful. With carbon dioxide dosing your plants will use and require more macro and trace additives and the next article will focus on why to use NPK and trace additives and the best practices on how to apply them to your aquatic garden.

  Advanced Aquarist, Advanced Aquarist
Jake Adams

 Jake Adams

  (26 articles)

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