Anyone looking to start a planted aquarium is going to run into the question of carbon dioxide (CO2) at some point. Should I be using CO2 on my tank? And if so, how do I set it up? Carbon dioxide is a somewhat complex subject because there are many ways to go about adding it to your aquarium. But it’s one of the best ways you can see a rapid boost in plant health and growth.
Why Should I Be Using CO2 on Planted Aquariums?
Carbon dioxide is one of the most important elements for growth in all plants, both aquatic and terrestrial! All organisms need a source of carbon to grow. Carbon is used both as food for plants, converted into sugars that provide energy as well as structural sugars (cellulose) that allow them to grow larger.
The greater the carbon availability, the more energy plants can produce for themselves and the faser they can grow, and vice versa. So supplementing CO2 is the fastest way to see vigorous plant growth. You’ll still need to provide good lighting and nutrients like nitrogen, potassium, iron, and other fertilizers.
But these are much easier to come by than carbon dioxide in most systems. While fish, shrimp, and other animals do constantly exhale small amounts of CO2 there’s never enough to make a tremendous difference in plant growth compared to providing it yourself.
Some plants, particularly low light plants, may grow too slowly to make much use of extra CO2. Anubias, Java Fern, and other species can make do without. But if you’re looking to grow fancier plants, especially tissue culture, stem plants, and carpeting plants, CO2 is almost mandatory for these species.
Sources of CO2
Now that we’ve learned why carbon dioxide is so important to plants let’s look at how to find it for them! As it turns out, we have a few choices in how to get CO2 into the water column.
I’ve discussed DIY CO2 systems in great detail before. These are a simple, affordable way to dip your toe into the world of carbon dioxide supplementation for planted tanks. This particular DIY setup uses yeast and sugar water in a fermentation process!
The yeast feed on the sugar and “burp” out carbon dioxide as a waste product. Over time, the pressure builds up, allowing CO2 to be pushed out of the bottle, into the lines, and into the aquarium. There, a diffuser stone helps break up the flow, creating a slow stream of bubbles that partially dissolve into the water for plants to then use.
Yeast reactors are fascinating and do provide enough CO2 for smaller tanks (15 gallons or less). However, they tend to run out rather quickly because the yeast also releases alcohol, which eventually poisons them. You then need to start the system over from scratch with fresh yeast and sugar. Continually fluctuating CO2 levels also tends to cause algae to explode in growth as plants struggle to compete with them.
Another DIY setup you can try is to use a citric acid and baking soda reactor. This particular setup is easier to control and maintain a consistent level of CO2 with.
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For most aquarium CO2 systems you’re going to be working with pressurized sources of carbon dioxide. This means handling metal canisters of varying sizes and bleeding off CO2 slowly into your tank. Pressurized sources of CO2 range from tiny bicycle tire refill cartridges to canisters several feet high. The majority of aquariums will be able to use canisters ranging from small refill cartridges to paintball gun sized canisters.
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If you’ve purchased a CO2 diffusing unit from a manufacturer like Fluval, they may offer their own proprietary CO2 canisters. But these are always massively overpriced and a terrible option. Bicycle refill canisters are the same size and you can get a year’s supply for the price of a single “name brand” CO2 canister. Just be certain that the threading on the regulator is sized to fit the CO2 refill canister!
Lastly, we always have natural CO2 as an option. Adding carbon dioxide is one of the surest ways you can boost plant growth in an aquascape. But there is always CO2 available in the environment for plants to use.
The problem is that the levels of carbon dioxide in the average aquarium are much too low for adequate plant growth. In the atmosphere there is around 400 parts per million of carbon dioxide; far more than enough for most plants. But thanks to the partial gradient between the air and water, you’ll typically only see levels between 2 to 4 ppm in most aquariums.
So how do aquatic plants get the CO2 that they need in nature? The decay of organic matter in the substrate is one source of carbon dioxide that boosts these numbers somewhat. But gravel and sand doesn’t contain any organic matter by itself.
You can use soil instead of sand or gravel, which then boosts the CO2 levels in your tank. This isn’t really practical unless you’re running a Walstad aquarium, where the plant growth can keep pace with the decomposition to filter out any pollutants that might harm your fish.
In nature the water is also being churned continually. Rapids, waterfalls, and barriers like driftwood or rocks can all help air get forced into contact with the water where CO2 can then dissolve into it, increasing the concentration for plants downstream to absorb. Since aquariums are much smaller in volume than any natural body of water this isn’t really a practical solution.
Measuring CO2 Levels
It’s extremely important to keep CO2 levels as stable as possible when running any kind of system on your tank. Fluctuating levels of CO2 aren’t harmful for plants or fish so long as they stay within healthy ranges.
But the problem with changing CO2 levels is that algae is constantly in competition with plants. Both of these organisms use a specialized enzyme called rubisco that allows them to take in carbon dioxide in the first place. Since rubisco is an expensive molecule to create, they prefer making just the right amount of it to suit the available levels of CO2.
If you suddenly change the CO2 levels – or never establish any kind of consistency – you can end up shocking your plants. Algae, on the other hand, can alter their ability to use CO2 much more quickly than higher plants. A sudden rise in CO2 can cause algae to take off because the plants aren’t using it. This is less of an issue compared to a sudden drop in CO2, though. This frequently causes algae blooms because your plant’s growth stalls. But there are still plenty of nutrients and light available so algae takes up the excess.
CO2 Drop Checker
A CO2 drop checker is commonly used to provide a constant measurement of the carbon dioxide levels in your tank. They are essentially a liquid test kit that measures the pH of the fluid they contain. Carbon dioxide dissolves from your water into the trapped air and reagent liquid, altering its color over the course of an hour.
With any CO2 drop checker, blue indicates that there are very low levels of carbon dioxide. As the levels increase, the liquid will turn a vibrant green color. These devices include a color chart that you can match the hue to, in order to tell roughly the concentration of CO2 in your tank.
You always want to have some idea of the CO2 levels in your tank because it’s easy for a plugged diffuser, spent CO2 canister, or other issue to go unnoticed for a long time. And if you have fish or invertebrates living alongside your plants, you’ll need to make sure that the levels don’t reach dangerous levels. The drop checker will turn a vivid lime green and you may or may not see fish twitching and gasping at this point.
The reagent liquid within the unit needs changing roughly every two weeks. However it only takes a few drops to refill, giving you months between needing to buy more drop checker solution.
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The second way we can track carbon dioxide as it enters the system is to use a bubble counter. How effective bubble counters are depends on their placement as well as the system they are made a part of. Typically, you’d use an in-line bubble counter, which contains a small amount of water.
As CO2 leaves your source and travels up the line and into your tank, the water contained in the counter allows you to gauge the flow by creating visible bubbles that you can count. There is no exact number of bubbles you should aim for because it depends on how efficient your diffuser is, your water flow, temperature, plant usage, and other factors.
But a rough rule of thumb for tanks 20 gallons and under is to start with one bubble per 4 seconds and give your tank an hour to saturate with CO2. You should also be using a drop checker alongside the bubble counter to gauge the saturation level as well as the incoming pure CO2.
Bubble counters can also be found integrated into your CO2 regulator, if you have one. But more passive methods of CO2 diffusion may not use or need a counter.
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Diffusing CO2 into a Planted Aquarium
Once you find a source of CO2 and a way to measure it, we have to then get it out of our pressurized source safely and in a form plants can utilize.
CO2 regulators are essential for medium to large planted setups where passive CO2 diffusion methods simply won’t work. We need large amounts of CO2 to provide a steady stream of bubbles into the tank. Therefore, you should be looking at pressurized CO2 sources, from paintball sized canisters to larger.
Since the CO2 levels in these containers are dangerously pressurized, we need a device that will let us safely bleed off enough for a steady stream of bubbles. A CO2 regulator is a powered device that uses an electric solenoid and a needle valve to do precisely this. They also include gauges that let you see the internal and sometimes the outgoing gas pressure so you know when to make adjustments and when to refill your canister.
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A CO2 regulator screws directly onto the CO2 canister, which can then be placed onto the floor near the aquarium and a power source. Lower quality units can sometimes run hot to the touch, so be careful when handling your regulator at first.
Good quality regulators often have a bubble counter mounted directly onto them rather than requiring you to buy an in-line model. Some also have a check valve built in. Check valves are essential for any tubing running into your tank. During a power outage it’s possible for water to reverse in flow, emptying your tank just like a gravity powered siphon hose does. A check valve ensures gases and liquids only flow in one direction.
CO2 Diffusers, Atomizers, and Reactors
Now that we’ve covered how to find and release carbon dioxide into your tank, we still need to make it actually accessible for your plants! Fortunately, we have diffusers, atomizers, and reactors to do precisely this!
Each of these does more or less the same thing: breaks up the stream of carbon dioxide into bubbles (or molecules) small enough to either remain submerged or for plants to uptake directly. But they vary in effectiveness and usage, so it’s good to discuss them separately.
A CO2 diffuser is the most common way to get carbon dioxide into the water column. It works the same way an aquarium aeration stone does. A piece of porous stone is placed on the end of a plastic, metal, or glass device that runs from your CO2 tubing into your tank. Many CO2 diffusers have a bubble counter built into the base, allowing you to gauge the flow of gas into the water.
As the gas hits the stone, it’s broken up into thousands of fine bubbles. These bubbles are still usually too large to stay submerged and float towards the surface as a result. Therefore, you always want to place the flow of your diffuser near either the intake or outflow of your filter. That way the bubbles are forced into staying submerged longer, allowing more CO2 to dissolve into your water.
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A CO2 atomizer is similar to a diffuser except it’s designed to create much smaller microbubbles. These super fine bubbles don’t immediately float to the surface. Here’s a video of the output of a CO2 atomizer. Notice how fine the output is and how they tend to drift along with the currents.
Atomizers can be both attached to the end of CO2 tubing the way a diffuser is or set up using an in line design. In-line atomizers are added to the outflow of a canister filter, merging your filter’s water flow with your CO2 system to send clean, carbon-rich water into your tank. I prefer in line atomizers because they are much tidier in appearance!
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Lastly, we have CO2 reactors as a way to diffuse carbon dioxide into a planted tank. Reactors are designed to be as close to 100% efficient as possible. They hold the CO2 bubbles inside and continually churns the water, breaking up even microbubbles repeatedly and forcing the CO2 into solution. The enriched water is then sent directly into the tank with no chance of bubbles leaking back out into the atmosphere.
CO2 reactors are especially handy for large tanks where we want to get as much carbon dioxide to our plants as possible. Some wasted gas isn’t so important for smaller tanks where even a standard diffuser attached to a canister will provide you with months of output.
It’s worth mentioning that some plants actually prefer the bubbles provided by atomizers and diffusers over reactors, however. Reactors are also much larger than diffusers and atomizers.
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Lastly, for any carbon dioxide system for a planted tank, you’ll need to use CO2 tubing. You might be tempted to use standard silicone airline tubing since it’s cheaper and you may even have some on hand right now.
The problem with it is that it isn’t fully impermeable to carbon dioxide. In other words, CO2 will leak right through the walls of the tubing as it runs into the tank. You can lose around 30% of your CO2 in this manner so you’ll need a few feet of CO2 tubing. This is typically colored black to help you tell the difference from standard tubing as you work around your aquarium!
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When Do I Provide CO2 to Plants?
It’s important to talk briefly about when one should run a CO2 system for plants because they don’t need CO2 24/7. In fact, it’s dangerous to run a carbon dioxide reactor all of the time, especially for your aquatic animals.
Plants only need CO2 when they’re actively photosynthesizing, meaning when the lights are on and you want them to be growing vigorously. It’s best to start your system an hour before the lights are scheduled to come on. You can set your CO2 regulator on a timer that powers it on an hour before the lights turn on.
This way, CO2 starts diffusing into the water column and hits peak saturation right as the lights click on. This way, your plants hit the ground running and photosynthesizing, preventing algae from getting a chance to compete.
Likewise, we want to shut off our CO2 system around an hour before the lights go out. The aquarium plants then mop up the remaining CO2 in the system and your fish aren’t left in a dangerous position.
The danger to aquatic animals in a CO2 enriched tank is at night when plants switch from producing oxygen to consuming it. That’s right; they consume oxygen and release CO2 at night. And if you don’t account for this, you may end up with massive, seemingly mysterious animal deaths overnight.
Carbon dioxide is an important nutrient that all plants require. Unfortunately, there just isn’t enough of it in unenriched aquariums to fuel fast plant growth, especially for stem and carpeting plant types. CO2 systems aren’t super expensive or difficult to run, however, and are the fastest way to ensure good health for your aquatic garden, and by extension, your fish!