How to Add CO2 – Introduction to Aquarium Plants Tutorial

Art PennomAquarium Plant Guide, Beginning Aquascaper, Blog8 Comments

How to Add CO2

This is version 1 of this article. I am asking for comments and feedback on it so that I can improve it. Thanks!

In parts 1 (Light Basics) and 2 (Light Basics Continued) of the Aquarium Plants Tutorial, we discussed the first part of the golden equilibrium- light. In part 3 (CO2 Basics), we introduced the second part- CO2. This part 4 gets into the practical aspects of adding CO2 to your aquarium. Let’s get to it!

You made me wait for this so let’s get right to the point. How should I add CO2 to my aquarium?

OK, OK, sorry about the wait but it’s important for you to digest parts 1-3 before diving into the various ways to add CO2 into your aquarium. Understanding why it’s important and what it does will make it easier for you to decide on how you will add CO2 to your current and future aquariums.

Step #1- decide on how much maintenance you want to do

Are you the type of person that has the time and inclination to tweak, fiddle with, prune and otherwise fuss over your aquarium on a weekly basis? You want flexibility in keeping many types of plants and you don’t mind putting in some of your time in exchange.


Are you the type that wants to do as little as possible because you’ve got other priorities? You want something that looks nice but you will exchange flexibility and many plant choices for ease and simplicity.

No judgment Either is fine with me. However, YOU need to accept a choice and accept it at this point.

Depending on whether you are one type or the other, the methods of supplying CO2 will vary.

Of course, you will be able to change in the future if you want.

Step #2- determine your budget and consider your ability to save. It will pay off in the long run.

Once you’ve decide on what type you are, at least for this aquarium of yours, then you need to determine what your budget is. Like many things in life, a higher price tag may buy you convenience.

If your budget is tight or non-existant, then you may want to focus on an aquarium that does not have a moderate or high CO2 need. We will discuss these in a bit.

Let’s explore some options:

I would prefer plant choice and flexibility and don’t mind some extra work

Wonderful, then you need to provide enough supplemental CO2 to so that the needs of the plants you choose are met. Once you find this amount, your system needs to be able to provide that level on a consistent basis.

Best Solution-  A pressurized CO2 system consisting of a CO2 cylinder, a good regulator, a solenoid valve, a timer, a needle metering valve, a bubble counter, CO2 tubing, a check valve, a drop checker and a CO2 diffusing system.

Semi Solution A- A semi-DIY citric acid and baking soda CO2 generator. This item is being sold via Amazon and has some enhancements over the old school DIY yeast and sugar CO2 system. It contains a safety valve that will release the pressure preventing nasty cleanup accidents. The citric acid/baking soda combination is said to be more stable than the yeast/sugar mixture.

Semi Solution B- A DIY yeast and sugar CO2 generator. There are many, many instructions for constructing this system on the Internet. It’s been used for years and it does provide supplement CO2 to your aquarium. The problem with it is that the mixture can be finicky, there usually is no safety valve in case of overpressure and CO2 delivery cannot be stopped at night.

My recommendation for you, however, is to go with the pressurized system. Yes, it’s expensive upfront, but it will make your life much easier in the long run. After having done semi solution B for many years in the past, I can tell you with all honesty, save your money and purchase the pressurized version.

OK well what if I prefer ease and simplicity and don’t care very much for flexibility and plant options. I just want something that is low maintenance and looks nice.

Great! Then you have options as well.

Best Solution- Although it’s not an absolute necessity, I would still encourage you to purchase a pressurized system like the one above if you can afford it. You would simply tune it down to provide a smaller amount of CO2 as your set up won’t require much.

Semi Solution A- A DIY system like the two discussed above can also work. The need to hit a certain range of CO2 isn’t there so any supplemental CO2 should be sufficient provided your setup is correct.

Semi Solution B- Going the natural route is now an option for you.

Remember in part 3 (CO2 Basics) we discussed the natural ways to get CO2 into the aquarium? They were fish food/poop, bacterial respiration and surface exchange. Due to the low CO2 demand of your aquarium, these three ways may be sufficient to hit the CO2 requirement.

If you will go this route, I would encourage that you ensure you have a healthy bacterial population by adding a carbon source such as vodka or vinegar. Alternatively, using a substance that provides plants with an alternative to CO2 such as Seachem’s Excel may be something to consider.

The important point with this option is that you MUST choose plants that have a low demand for CO2 in the water column. Plants such as anubias, cryptocorynes, and ferns.

Hmm, come to think of it, I think I prefer to save up for a pressurized system. What I don’t have clear is how it works!

OK great!

Now that you’ve made a conscious decision about what you want and why, we can certainly get into some details about what’s important for you to know.

So, let’s start with the cylinder itself. They come in a variety of sizes from desktop paintball gun cylinders to large 20 pound or more industrial cylinders. Obviously, the smaller it is, the more frequent you will have to replace or refill it. Then again, a huge 20 pound metal object may not look as nice or may be overkill for your zen-like 5 gallon tank.

The paintball type cylinders come full of CO2. They are discardable once they get used up and you purchase another one.

The larger tanks are refillable or at least swappable. You can get them refilled at dedicated CO2 providers or soda dispenser vendors.

When you go get your first one or a refill, you will see that it will be cold and get condensation on it. That’s fine. It’s normal.

When you get it into the car, please place it in a secure location where it won’t accidentally tip over. I would also recommend that you drive with the windows open. There have been reports of the safety valve going off with the windows up in a car…

Picture this, you’re driving along, happy that you’ve got the CO2 coming to make your plants grow and PUFF! Next thing you see is all white and you feel very cold. You can’t see in front of you. It’s all you can do to open your window and stick your head out. No, let’s avoid that. Keep the windows down.

Yikes! Is this thing safe?

Well, pretty much but we need to be careful. Right? It’s a cylinder that is pressurized after all.

When you get it home, put it in safe place. Likely it will be in your aquarium cabinet. Some people use a safety harness to hold it in place. If you have little ones, make sure to put a child-safe lock on the cabinet door.

If you exercise caution and respect, then you’ll be fine.

OK What’s next?

Now you want to connect your regulator to it. Just screw it to the cylinder and make sure to use one of the plastic o-rings that came with it. We have discussed CO2 regulators previously in a ScapeFu Podcast episode.

Of course, you should already have connected the regulator to a solenoid valve. The solenoid is already connected to a needle metering valve, which, int urn, is connected to a bubble counter. The bubble counter is then connected to the CO2-safe tubing.

A check valve will keep water from flowing back into the regulator system. Then you’ll have some sort of CO2 diffusion mechanism. I’ll explain more about how to best diffuse CO2 into your aquarium in the future. For now, I’m assuming you have some way to get the CO2 into the water.

Simple, right?

Not really, but OK. I think I can figure the hardware out eventually. But how does it work? How do I know how much CO2 to use?

Great question!

This is called “dialing in” your CO2. It’s all about finding the correct amount of CO2 for your particular aquarium. It will take some work so you need to have patience. I strongly recommend that you start it when you will have a week or two when you will be able to tweak your settings.

But! Before we get started, we need to understand our water parameters.

What? Why?

Well, we are going to be dissolving CO2 into the water. CO2 will decrease your pH as it forms acid in your aquarium.

You remember the formula, right?

CO2 + H20 <-> H2CO3 <-> HCO3 <-> CO3

Adding CO2 will push the equilibrium right which will decrease the pH because you will be creating acid. Removing CO2 will push the equilibrium left increasing pH by taking up H+.

This is all well and good but if we don’t have enough pH stabilizer a/k/a alkalinity in the water, you may experience some wild swings. Your plants won’t like it but CERTAINLY your fish, inverts and bacteria will hate it. Remember that even small pH changes can create a big change in the water’s acidity.

Alkalinity makes changing the pH harder when you add CO2. This is a good thing to keep our ecosystem steady. Therefore, your water should have sufficient alkalinity for this purpose.

Many freshwater test kits refer to this alkalinity as KH or carbonate hardness. It measure the amount of bicarbonate and carbonate in the water. We are looking for about a 4 dKH or about 72 ppm.

This is for softwater plants. If you choose to use hardwater plants, then your GH and KH should be higher. However, for our purposes here, I’m going to assume that you want to use softwater plants that represent a high percentage of what’s available out there.

So, test your water and see where you are with alkalinity. I use the very awesome Hanna Alkalinity Meter for Freshwater. If you need to adjust it, you can use any commercial product designed to increase KH. You can also use backing soda to raise KH without too much of an effect on pH. If you need to bring it down, then you will need to find a way to soften your water.

Now we are going to rely on the very cool sounding drop checker to help us dial in our CO2. As I mentioned, there is no magic number that works for everyone. In fact, it’s more about a range than a specific number. We want to provide enough CO2 so that are plants are photosynthesizing optimally without making the fish, shrimp and bacteria uncomfortable.

How will we do it? Slowly.

Even though your test shows carbonate hardness (alkalinity) of 4, there is no way for us to know how much of that is true bicarbonate/carbonate and how much of that is something else such as phosphate. Therefore, if we relied on the KH/pH/CO2 table, we would be off.

The beauty of the drop checker is that it uses a pure solution with a pure 4d KH and only allows CO2 to interact with it. It magically turns green when the solution reaches a pH range where a good amount of CO2 is present in the water. It is this green that we are after initially.

Go to your needle valve and start to turn it so that 1 bubble per second is generated in the bubble counter. Now go away for 10 minutes.

Go ahead. I’m waiting…

Now that 10 minutes have passed, take a look at the bubble counter. Is it still bubbling at 1 bubble per second? If it is, awesome! You seem to have a good, steady needle valve.

If you see that now you are off your initial setting, you will need to keep an eye on the needle valve. It may not be very accurate.

Assuming that the needle valve is holding steady and that we are now in the beginning of the photoperiod of the aquarium, go away again and come back in 3 or 4 hours.

I’ll be here. Don’t worry.

Three or four hours later

Welcome back! Go look at the bubble rate. Is it still 1 bubble per second? If not, adjust it to 1 bps.

Now, look at the drop checker. What color is it?

  • Blue-bluish – you need to add more CO2.
  • Green/greenish – we’re doing well so far.
  • Yellow/yellowish – we’re adding too much CO2. We need to tone it down.

OK So how do I turn down or up the CO2 and by how much?

You need to give that needle valve a subtle turn. Usually a quarter to a half turn. Sometimes less.

Honestly, I’m embarrassed to say it but my fingers are too big to really see if I’m turning it slightly. There’s got to be a better way?

Well… not really but we can make things a little easier.

If you’re lucky, your needle valve comes with a dial that will allow you to make one notch turns relatively easily. If you’re like most people, however, all you have is an incredibly small knob to turn with no guide.

If you are the latter, here’s what you can do to help. Go get a toothpick and some superglue. Place one drop of superglue on the top of the knob and then place the middle of the toothpick into the drop. Hold it there a few seconds and viola! You have a ghetto indicator that will give you an easier representation of how far you’ve turned.

So, now that you can see what you’re doing better, if your drop checker is in the blue area, turn the knob one quarter turn more in order to let out more CO2. Keep an eye on the bubbles. Don’t go over 2 bubbles per second.

If you are in the yellow, throttle it back one quarter turn to slow down the bubble rate. However, make sure you didn’t shut it off.

Now, go about your business for an hour and come back to check on the color again. You want to see a nice solid green color. Follow the same procedure again if you don’t see a color change to green in the next hour.

Why do I have to wait so long?

Sorry but the way the drop checker works, it takes about 2-3 hours to see color change. So, always remember that you have this lag time when changing CO2. That’s why we ALWAYS GO SLOW.

Once we get to a nice green color, smile, pat yourself on the back and close the cabinet door. You’re done for the day. You’ve finished the day by getting the CO2 level of your aquarium to a range that will start having positive effects on your plants without wiping out your fauna. Well done!

Let’s talk a little about that timer I had you buy. It is going to be connect to the solenoid valve. Make sure you’ve done that.

Now, decide at what time your lights will turn on and when they will turn off. Remember we learned about the photoperiod in part 1 (Light Basics) and 2 (Light Basics Continued) of this Aquarium Plants Tutorial.

Set this timer to turn on the CO2 1 hour before the lights come on and to turn off the CO2 1 hour before the lights turn off. This will allow the CO2 to build up in your aquarium and be ready to be used when the lights come on. It will shut it off when your plants are winding down for the day.

Got it! So that’s it?

Add CO2 rangeAh, no. Patience my newbie friend! You’ve only more or less set your CO2. You haven’t dialed it in yet.

Remember we said that each aquarium is different. We need to find the CO2 range that is optimal for your plants.

Before I tell you how to do it, let’s talk a little about information on the Internet, tables and CO2 controllers.

If you search, you’re going to find all sorts of information on the Internet about the “right” amount of CO2 to provide. You will see the famous KH/pH/CO2 table that depicts the amount of CO2 theoretically in your aquarium at a certain KH and pH. Lastly, you may see people telling you that a CO2 controller uses this table to maintain your CO2 levels at a certain point.

I don’t want to get into whether all this is right or wrong. To me, it doesn’t matter. I’ve tried it all. What does matter to me is that you really don’t need a CO2 controller or the KH/pH/CO2 table to dial in your CO2. If you want to purchase the CO2 controller because you love gadgets, please do. However, then understand it’s limitations.

Let’s now discuss how to dial in the CO2.

Once you wake up in the morning, head over to your aquarium about 1 hour after lights are on. What color is the drop checker?

If should be green. If it’s blue, you need to turn the dial a quarter more. If it’s yellow, turn it down a quarter.

Now, go away again and enjoy your day. However, please come back 2 hours before the lights turn off.

Flash to two hours before lights turn off

Welcome back. What color is the drop checker?

If it’s green, fine. Blue, turn a quarter. Yellow, turn down a quarter.

Please continue this daily process for at least three days.

What we are doing is trying to get that drop checker to be green one hour into the lighting period and to remain green throughout the photoperiod.

This may take a much as a week before you find the right balance of what your plants are taking up and what you are putting into the tank.

Tiny changes are used to minimize the chance that you introduce too much CO2 and kill your fauna.

What happens when I find this balance? Is it set it and forget it?

Well, once you find this balance, you are not done. You’re really never done because as your plants grow and mature, their CO2 uptake rates will change. However, unless something goes really wrong, you will never have to do the above steps again.

Now that we are in a range that is providing benefits to our plants. How do we find that optimal, sweet-spot where our plants are really growing at the best rate they can?

We use our power of observation. Yes, no magic. No specific number like 44 ppm CO2 or anything like that.

We will let our plants tell us.

You… Will… Become… The… Plant Whisperer!

Excited? Me too. Let’s go!

One fine morning when you can spend the day checking on your aquarium, you are going to turn that knob one quarter turn up in the morning. You’ll come back in 2-3 hours and check on the plants and the drop checker.

If you went slow, you will see the color is now greenish yellow. Your plants should look perky. Perhaps you’ll see some bubbles underneath leaves. You may have pearling- tiny bubbles coming from your plants.

How are your fish doing? Swimming normal or are they bunched at the top looking distressed?

If things look good, give another quarter turn. Stick around checking on your aquarium often now. We are pushing boundaries and we don’t want to go over the line for too long.

Keep an eye on the fish and plants. What you’re looking for is a lot of pearling which means your aquarium is super-saturated with oxygen from the photosynthesis process. You are also trying to make sure your fish are not suffering.

If you have a pH test kit or meter, use it now to make sure that your pH isn’t dipping too low. Check your alkalinity as well to make sure you won’t have unexpected and sudden changes.

During this phase, I go real slow. I normally wait 24 hours in-between quarter turns to allow the fish and shrimp to acclimatize to the increased CO2. There’s really no hurry. Your plants are getting CO2 and photosynthesizing well. Right now we are simply trying to find the optimal spot for them.

Once you see your fish in any type of discomfort, pull back by a quarter turn. Wait a little and make sure that the fish are back to normal. Once they are, you’ve found your spot. Leave it there.

Now, you can say you dialed in your CO2 optimally for your particular aquarium.

If you want, check the pH level and the KH and see how much theoretical CO2 is present in your aquarium. Understand that it won’t be precisely what the table tells you but you will have a rough idea.

Man, that was a long process! I’m glad I’m done with that! Now my plants will grow well?

It was long initially but once you find the sweet spot, you have a much easier time of it. However, we’ve only covered two parts of the golden equilibrium- light and CO2. You still need to learn about the third and final part- fertilization.

For that, however, you must wait for part five of this Introduction to Aquarium Plants Tutorial.

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8 Comments on “How to Add CO2 – Introduction to Aquarium Plants Tutorial”

  1. John Pini

    Good article I just had one comment: Paintball type Co2 canisters are refillable. I used to play speedball and I have my old 20oz paintball canister with an aquatek paintball regulator. I refill it about twice a year with it feeding 2 tanks. Tha paintball canisters are refillable, the mini canisters (such as fluval or ada) are disposable.


    1. Art Pennom

      Hey, John. Thanks for your comment.

      You’re absolutely right. The paintball cylinders are larger than the disposable canisters that I was thinking about when I wrote this.

      Best regards,


  2. somraj

    Hi Art, I must say the above literature is really helpful. Its being explained in the most simplified manner. Every beginner should go through this. Keep up the awesome work.

  3. Carey Cottle


    Thanks so much for your informative blogs and podcasts. I look forward to every posting.

    In your podcast on injecting CO2, and in this article, you and JJ expressed the opinion that you favored using a needle valve and bubble counter and drop checker to manage the amount of CO2 injected. In the podcast you specifically stated you did not value the use of a pH controller to manage the amount and timing of CO2 injection. Based on my experience and knowledge I disagree and here’s why.

    I have 2 heavily planted tanks. One is a 90 gallon tank and the other is the 16 gallon Mike Senske ADA tank from the AGA conference this year. You have seen it. Your suggestion of about 1 bbs is about right for the smaller tank but is way off for the 90 gallon tank. Clearly it takes much more CO2 to feed the plants in a 90 gallon tank than it does for a 16 gallon tank. A pH controller makes this much more manageable. The buffering capacity of the tank will hold the injected CO2 until the CO2 is used by the plants and then the pH rises telling the controller to inject more CO2. If this controller is combined with a timer so that CO2 is not injected at night when it’s not needed this safeguards the system. I still use a drop checker as a final safeguard.

    Where am I wrong?

  4. Kiam

    I am setting up my first pressurized co2 aquarium setup and return again for reference. This is not only informative but your writing style gives me a confidence boost that I can get it right. Thank you!

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