How much CO2 do aquarium plants need?

You’ve probably noticed that I’m a big fan of adding enough CO2 to aquariums. 🙂 I think I insist on its importance in about every article. And for a good reason. Next to phosphate, nitrate and potassium, CO2 is the fourth “macro-element” plants need for healthy growth.

After my article on how to add CO2 safely into your aquarium, I liked the idea of diving deep into how much CO2 your aquarium needs. You really don’t want to add too much, but too little is not good either!

Which factors determine how much CO2 my aquarium needs?

Let’s start with the simple stuff.

How much CO2 does your aquarium need? The general rule is that 30 ppm (30 mg/L) of CO2 is ideal for a planted tank. But every aquarium is unique (just like its owner 🙂 ), so here are some important factors that determine how much CO2 your aquarium plants really need.

1) Light intensity above your aquarium

No doubt your aquarium is well-lit, as the plants in your aquarium need light. In addition to light, these plants also need CO2 (and plant fertilizers) for their photosynthesis.

The more light your plants receive, the faster they grow and the more CO2 and fertilizer they need!

So, if you want your plants to keep growing, you’ll have to add more CO2 and other nutrients. If you don’t, they’ll stop growing, start melting, you’ll get ugly algae, water quality will plummet… everything you don’t want!

2) Number of plants in your aquarium

Taking what you’ve just read above, it’s also logical that the more plants you have, the more CO2 your aquarium needs.

On top of that, certain types of plants need more CO2 than others, especially slow-growing plants. For example, while Anubias can survive easily in an aquarium without CO2, Hemianthus callitrichoides won’t make it.

3) Amount of other nutrients in the aquarium

And this takes us to the third point… If you add fertilizers, they make your plants grow faster. In turn, this increased growth increases the CO2 demands.

This is also the reason why lots of people mistakenly believe that fertilizers cause algae growth. They already have a small CO2 deficiency, they add fertilizers which promote plant growth (increasing the CO2 demand), and the CO2 deficiency only becomes worse. The trick is then to solve the CO2 deficiency and add fertilizer as well. Only then, your plants will thrive.

OK, so, how much CO2 does my aquarium need? How do I start?

Good point. In general, I recommend starting with a given number of bubbles per second and then adjusting the flow very slowly and carefully until your plants grow nicely without your fish and shrimp to get stressed. I usually recommend starting with 1 bubble per second for a 100-liter tank. Bigger tanks need more and smaller ones need less.

“1bps for a 100-liter tank”

Also, take the above points into account: how much light do your plants receive, do you have few plants or plenty of them, do you add a lot of fertilizer?

How to measure the CO2 level in my aquarium?

There are various ways to measure the CO2 content of your aquarium, which all have their advantages and drawbacks.

Measuring CO2 using the pH/KH chart

Although the pH/KH chart itself is correct, I think it’s not usable for aquariums. It actually assumes that the water only contains carbonates, which is rarely the case (think about the tannic acids from your wood for instance). Therefore, you get anomalies and the table suggests CO2 levels that are way higher than in reality, often leading to CO2 deficiencies.

Measuring CO2 using a drop checker

The drop checker is much more accurate, you can practically always trust it. It’s just a little slower and sometimes a bit hard to read. I’ve already written an article with all kinds of tips to make the best use of your drop checker, you can read it here.

Measuring CO2 by looking at your plants

The plants themselves are my favorite, you just look at your tank! If your plants grow well, then you are adding enough CO2. If you see malformations, algae and so on, then it’s a good idea to carefully increase your CO2.

Where to refill your CO2 cylinder?

So, have you found out the ideal amount of CO2 for your aquarium? Perfect!

Now, you really don’t want to run out of CO2. I get this question very often: “where to buy a CO2 cylinder for my aquarium or refill my CO2 cylinder?”. Well, I usually go to a gas retailer. You pay a deposit for the CO2 cylinder, which you can get back if you ever quit your hobby. Depending on the retailer, either you get a refill or you bring your old cylinder in exchange for a new one.

As an alternative, you can go to your local aquarium shop, but it will be more expensive. Some other options are homebrew supply stores, food products suppliers and paintball stores. They can also provide you with CO2 cylinders.

Conclusion

Keeping an aquarium is no exact science, which probably doesn’t come as a surprise. It’s a continuous learning process which can give you a breathtaking aquascape in return. With some time, love, attention, and a CO2 drop checker, you will get there.

Here are a few things to pay attention to:

• Is your aquarium weakly or strongly lit?
• Do you have few or many plants?
• Do you add a lot of fertilizer?

I recommend you start with 1 bubble of CO2 per second if you have an aquarium of about 100 liters. Next, you can adjust the CO2 flow by looking at how your plants and fish respond. But as always, be careful and change the CO2 flow in very small steps! Do you have questions? Let me know below in the comments, I’ll be happy to help you 🙂