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The invisible culprit in your aquarium

Have you noticed?

Sometimes your aquarium plants still don't come out so well or they lack a little color. Even if you already add enough fertilizer with my All-in-One or Estimative Index Kit.

Well, the culprit isn't your fertilizer, or your lighting, or your soil, ...

No, the culprit is invisible: CO2!

CO2 is one of the most important nutrients for your plants. It is called a "macro element" for a reason. This applies to the trees around your house as well as to the plants in your aquarium.

Unfortunately it is also one of the most difficult nutrients to add to your aquarium....

In this article I explain some of the principles of CO2, as well as some links to useful articles that may help you further.

Is CO2 even necessary for your tank?

Every tank benefits from CO2. But sometimes it is not necessary to install a CO2 system. It depends on your goals.

A tank with simple aquarium plants that can easily handle dim light will probably survive on the naturally occurring CO2 in your aquarium. Here’s an example of a no CO2 tank I made for my dad a long time ago:

But you want nice foreground plants, strong colors, fast plant growth, … ? Then I definitely recommend you to go for a CO2 system as this will help you get the growth you want.

Both methods have their advantages and disadvantages:

Without CO2: less work and usually less algae BUT your plant choice is severely limited and plant growth is very slow
With CO2: more work BUT faster plant growth, more plant choice, more color in the plants, better growth, …

If you are interested in an aquarium without CO2 I recommend that you read this article. If you do want to use CO2, read on.

Which CO2 system for your aquarium?

OK, so you’ve decided to add CO2 to your tank. But what exactly do you need? A standard CO2 system consists of:

A refillable CO2 bottle. The larger the better;
CO2 pressure regulator which you connect to the CO2 bottle and reduces the pressure of the bottle to a workable pressure;
A solenoid valve that turns off the CO2 at night. Your plants don’t need CO2 at night;
A needle valve to finetune the amount of CO2 you add, to the drop.

This will get you a long way, but you still need to distribute the CO2 around your aquarium. And that brings us to the next point …

CO2 reactor, atomizer of diffuser?

You generally have 3 ways to mix CO2 into your water:

CO2 reactor: this is a kind of tube (see the picture below) into which the CO2 is pumped and then dissolved by the flow and “obstacles” (e.g. filter wool). This method is extremely efficient but can slow down your filter;
CO2 atomizers: with an atomizer the CO2 is forced under high pressure through a ceramic tube, causing 1 large CO2 bubble to transform into hundreds of small bubbles that are swept along in the flow, improving the dissolve rate. An advantage is that they do not obstruct the flow of your filter. A disadvantage is that they can create a bit of “fog” in your tank;
CO2 diffusers: this works the same way as an atomizer, but most are made of glass (do check our Pro Diffuser which is made of plastic AND is even more efficient). This makes them look particularly beautiful, but they do need to be cleaned more often.

If you want more info about the pros and cons of each CO2 system you can also read my article on AquascapingBlog.

In addition, you also have some useful accessories for your CO2 system: a good CO2 hose that can handle the high pressure, a bubble counter, possibly a drop checker …

Pro CO2 Atomizer

How do you know how much CO2 you have in your aquarium now?

Ha, good question. There are several ways …

Most people use the pH/KH table, but to be honest I don’t recommend it. This table assumes that your aquarium water consists only of carbonates but that is not the case.

There are all kinds of acids and tannins that influence the pH or KH in your aquarium but do not add CO2. This causes the table to give a much higher CO2 value, while in reality the CO2 is a lot lower …

pH KH tabel

A better way is a “drop checker“. I recommend you to read this article, there you will find all the information about how this device works and how it can improve the health of your tank.

drop checker kit aquarium

How do you safely add CO2 to your aquarium?

OK, you have everything you need to add CO2. Then please hold on, don’t start adding CO2 at random immediately. Too much CO2 can be toxic to your fish.

It is best to start with a certain amount of CO2 and then increase the CO2 slightly each week until you are satisfied with the plant growth. Do this at most 1x per week and always in very small increments.

Also don’t forget to keep an eye on the behavior of your fish after you have changed your CO2 setting. I have already written an extensive article on how to safely increase CO2 in an aquarium that already has fish in it. You can read it here (click).

When to start adding CO2?

Here’s a super handy tip for you: start adding CO2 at least 1 hour before the lights come on. This is because your CO2 needs to dissolve for a while before it is available to your aquarium plants. By starting a little earlier, you can be sure that your plants will be getting plenty of CO2 as soon as the lights come on and photosynthesis begins. Think of it as a hearty breakfast for your plants 🙂

In the evening you can stop CO2 again 1 to 2 hours before lights out. Usually the demand for CO2 will be lower then and there will always be some residual CO2 available in the water.