In parts 1 and 2 of this Introduction to Aquarium Plants Tutorial, we discussed the first part of what I call the golden equilibrium for planted aquariums – Light = CO2 = Fertilizer. You need to find the golden equilibrium for you particular setup. Once you do, you will have a beautiful and healthy aquarium where algae is minimized and plants are growing well.

Part 3 of the Aquarium Plants Tutorial is all about the middle part of the golden equilibrium- carbon dioxide (CO2). Really, what we’re talking about is giving the plants what they need to be able to photosynthesize.

Get it?

Well, not really, but OK. I understand why plants need light. Honestly, I don’t know why they need CO2.. uh, I mean, DIC… uh, that sounds funny. You know what I mean!

Yes, I do know what you mean and it too sounds funny. Let’s just say “CO2” and avoid confusion. That’s accurate for the majority of cases.

So, let me explain.

Last time, we learned that light + water + CO2 is what plants need for photosynthesis?

The formula was 6H2O + 6CO2 ————> C6H12O6 + 6O2.

So, without getting too deep into the weeds in this tutorial, all we need to care about is that for plant photosynthesis, we need light AND CO2. Light, without a source of CO2 means there is no photosynthesis.

Wait, wait a minute. I’ve seen on the Internet that CO2 is not necessary to grow plants. Some very smart lady even wrote a book on it and called it something like “El Natural”. What gives?

You’re on the ball today!

Yes, yes. I don’t mean to imply that YOU, the aquarist, must supply supplemental CO2 to grow plants. However, what I am telling you is that SOMETHING must supply CO2 into the aquarium so that your plants can photosynthesize.

What something, you may ask?

Well, CO2 can get into the water column in a number of ways:

1. Gas exchange at the water surface with the air around the aquarium. However, this is a slow process that doesn’t supply much CO2 into the water.
2. Fish can provide CO2 by way of respiration. Also, fish poop and food decompostion can also provide CO2. Like above, this doesn’t provide a lot of CO2 into the water and it can be an unstable source.
3. Bacteria and organisms in the substrate (and all over your aquarium) can also generate CO2 through a number of processes. How much this can contribute to the water column is hard to tell, but it isn’t much.
4. Supplmentation of CO2 by the aquarist. This is probably the easiest way to get CO2 into the water column in a way that is controlled. We’ll talk more about this later in this section of the tutorial.

OK, that makes sense. CO2 needs to get into the water somehow. Anything else that I need to take into account?

Well, yes.

The type of dissolved inorganic content that exists in your aquarium water is dependent on the water’s pH. You remember pH from school, right? It’s a scale of how acidic or alkaline something is? Right.

Look at this graph:

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As you can see, depending on the pH of the water, DIC will be a combination of CO2, CO2 and HCO3, HCO3, HCO3 and carbonate (CO3), or just CO3. In other words, CO2 will transform some or all of itself into bicarbonate (HCO3) or carbonate (CO3) depending on the pH of the water.

So I need to worry about having the proper pH in my aquarium? Are HCO3 and CO3 bad?

I wouldn’t use the term “bad”, but… Aquarium plants, for the most part, prefer to take up CO2 to fuel photosynthesis. It’s easy for them to do so in terms of energy expenditure.

Some plants that come from areas of very hardwater (i.e., high pH) have evolved the ability to use HCO3 instead of CO2. This comes at a higher energy cost to the plant. Softwater plants that come from a low pH environment where HCO3 are very low have not developed this type of mechanism. Most aquarium plants fall into the softwater category.

So… depending on your choice of aquarium plant, it may be very important to maintain a proper pH level so that most of the DIC is in the form of CO2.

Of course, if you’re going to target a pH of say 6.3, like I do, then your choice of fish and other fauna should be able to live comfortably in such a pH level. Some fish don’t do well in the environment.

Are you starting to see how, in a planted aquarium, things are all related?

Yup! It’s starting to make sense. It’s not hard once you understand it. So, I know my plants need a source of CO2. And, I know I want to keep softwater plants that will prefer available CO2 in the water. What’s the best way for me to get it in there?

Well, here’s really where you have options. I think it’s based on the type of aquarium you want to have and, of course, your budget.

As we learned above, there are a number of ways to get CO2 into the aquarium. We can divide them into indirect methods (e.g., via fish poop) and direct methods (e.g., a pressurized CO2 injection system). Which one to go with will be dictated first by your choice of aquarium. Here’s what I mean.

If you tell me:
“Art, I would like to have an aquarium that features nice slow-growing plants (like anubias and cryptocorynes) and I don’t have the time to be trimming plants weekly.”

I would tell you:
“Great. You can certainly get by using the indirect CO2 methods with that setup. You will correspondingly lower your light levels and fertilization routine.”

Which method is up to you but I would suggest that all the indirect methods listed would work together to supply enough CO2 for your plant setup.

A word of caution, however!

In my opinion, setting up a so called “Dirt tank”, Walstad El Natural tank, mineralized soil based tank, etc. can be rather difficult for beginners. There are just too many variables and things that can go wrong. Algae is a constant threat and despair shall surely follow.

My recommendation, stick to the basics outlined above as indirect methods and chose your plants wisely. Meaning, choose low-light plants such as anubias, cryptocorynes, ferns, etc.

OK but if I tell you I want to have flexibility to have a nice green “lawn” and healthy, big red plants, what would you tell me?

In that case, my friend, I would say, “Welcome to the wonderful world of direct CO2 injection!”

Great! Thanks! How do I go about setting it up?

Well, for that you need to wait for Part 4 of the Aquarium Plants Tutorial creatively titled, “How to add direct CO2 into your aquarium.”

Oh. great…