CO2 diffuser aquarium: the complete guide

If there is one thing that truly makes me happy, it’s aquascaping tools that are both functional and simply look great in your tank.

That is definitely the case for aquarium CO2 diffusers. You know them, those beautiful glass additions to your CO2 system that spread stunning tiny bubbles throughout your aquarium. But how does a CO2 diffuser actually work, and how do you get the most out of it? I wrote this aquarium CO2 diffuser guide so that from now on, you’ll know absolutely everything about your favorite diffuser.

An aquarium CO2 diffuser is usually a small glass device with a white membrane inside. You attach it inside your aquarium using a suction cup and connect it to your CO2 pressure regulator.

The CO2 from your regulator flows to the aquarium CO2 diffuser, where you’ll notice that the gas builds up in the small “chamber” beneath the membrane. As soon as the pressure in that chamber is high enough, the CO2 is forced through the membrane, creating extremely fine CO2 bubbles (unlike a CO2 atomizer, which creates more of a mist).

Those tiny bubbles spread through your aquarium like a kind of underwater ballet and dissolve into the water. Sometimes they stick to plant leaves, which actually helps plants absorb CO2 more efficiently. As mentioned, aquarium CO2 diffusers are usually made of glass, which makes them very attractive but also fragile. The upside is that you can immediately see when it’s time to clean it, as a perfectly clean diffuser is essential for optimal performance.

PS: many professionals also use an aquarium CO2 diffuser in their tank. Below you can see the tank of Adam Paszczela with a CO2 diffuser placed at the front:

There are many different types of aquarium CO2 diffusers available, so I’ll give you an overview to help you choose the best CO2 diffuser for your aquarium.

The good news is that choosing a aquarium CO2 diffuser largely comes down to personal taste. Broadly speaking, the differences between diffusers are mainly about style and, of course, capacity. In my shop, you can choose from:

• Regular diffusers or Cup diffusers: both types are made of reinforced glass and are mounted with suction cups. The difference is not so much in how they work, but in how they look. The Cup diffuser is also available for larger tanks up to 500 liters, while a Regular diffuser has a maximum capacity of 150 liters in the medium version. Both are so-called “in-tank diffusers,” meaning you place them inside the aquarium.
• Pro CO2 Diffuser: recently I added a “Pro diffuser” to the range. It’s made of plastic and uses a different membrane that works much finer. The big advantage is that it’s far less likely to break, and it’s also significantly more efficient thanks to the improved membrane.
• Edge diffusers: this is also an “in-tank diffuser,” but instead of attaching it with a suction cup, you hang it over the edge of the aquarium. It gives a very clean, minimal look.
• Inline diffusers: an inline diffuser is connected to the filter hose and is very suitable for larger tanks. Personally—being completely honest—I’m not a big fan of them because the risk of leaks is higher. A small crack in the glass and your tank could drain. For that reason, I only sell in-tank diffusers in my shop.

Do you still prefer something external? Then take a look at my CO2 atomizers.

How you connect an aquarium CO2 diffuser mainly depends on the model, but I’m happy to share a few basic guidelines. These are the key points to keep in mind for a proper aquarium CO2 diffuser setup:

1. Check which type of CO2 tubing you need for connecting your aquarium CO2 diffuser. Some diffusers require flexible tubing (for example, my Edge diffusers), while others work perfectly with rigid CO2 tubing (such as my Cup or Regular diffuser).
2. First, dip the CO2 tubing in boiling water and carefully slide the tube over the stem. The hot water makes the tubing more flexible and gives you a bit more room to work.
3. Place your CO2 diffuser opposite the filter outlet. That’s usually where the strongest flow is, so the bubbles are immediately carried through the tank and dissolve very efficiently.

Connect the other end of the CO2 tubing to your CO2 system. Be sure to add a check valve in between so water can’t flow back toward your regulator.

I’ve previously written an article about the amount of CO2 you should add to your aquarium depending on plant mass, lighting, … I definitely recommend reading that article carefully.

Once you know how many bubbles per second you need and your aquarium CO2 diffuser is installed as described above, you can get started. Turn on your CO2 system and you’ll see the CO2 slowly travel through the tubing to the diffuser. Next, you’ll notice the CO2 bubbles “dripping” into the diffuser chamber and forming a CO2 layer at the top. As soon as there is enough pressure (this can take some time, depending on how much CO2 you’re adding), tiny bubbles will appear. Your diffuser is now working perfectly.

PS: I’m sometimes asked, “why don’t I see mist?” This is a common misconception: a diffuser produces bubbles, while a CO2 atomizer produces mist. So this behavior is completely normal. My Pro Diffuser creates the finest bubbles.

Cleaning a CO2 diffuser sounds worse than it actually is. In practice, it simply becomes part of your routine during water changes.

Place the diffuser in a solution of 1/10 bleach and water and let it soak for about half an hour. During that time, I usually do my regular tank maintenance: trimming plants, changing water, removing spot algae from the glass, … After that, gently scrub everything with an old toothbrush or filter floss. Don’t forget to let the diffuser sit in a solution with water conditioner for a few minutes afterward to neutralize the bleach.

Rinse it off and you’re done! Below you’ll find an interesting video on this topic. This person uses a bit more bleach but compensates with more water conditioner.

Tip: if you clean your CO2 diffuser regularly (read: during your weekly water change), you’ll rarely run into clogging issues, and your diffuser will stay in top condition.

Why do the CO2 bubbles only appear in one spot on the diffuser stone?

I get this question a lot as well: the CO2 bubbles only come out in one spot instead of across the entire surface of the diffuser stone. Don’t worry—this is normal behavior for classic CO2 diffusers, and your diffuser is definitely not defective.

Why does this happen? The CO2 builds up under pressure and looks for the fastest way to escape, and in your diffuser that just happens to be that specific spot. You might see the same thing with another CO2 diffuser, but at the opposite side or more toward the center. This is not a sign of poor quality, just basic physics. Even ADA diffusers can behave this way. You can easily test this yourself: if you increase the CO2 output (more bubbles per second), you’ll eventually see CO2 emerging from multiple spots. That’s because the CO2 can no longer escape fast enough through that one point and starts looking for the next fastest route … 🙂

Prefer CO2 across the entire membrane? Then I recommend the Pro Diffuser, which uses a different type of membrane that produces finer CO2 bubbles.

Are you planning to buy an aquarium CO2 diffuser? Then there are a few things to pay attention to if you want to avoid a bad purchase.

• Buying a glass diffuser? Not all glass is the same. Choose a diffuser made from reinforced glass or laboratory glass. The last thing you want is for it to break while you’re cleaning it.
• Going for an in-tank diffuser with suction cups? Make sure the suction cups are strong enough. Cheap models often cut corners on suction cup quality, and before you know it, your CO2 diffuser comes loose.
• Choose the right capacity. Every diffuser indicates the aquarium size it’s suitable for. A bit of extra capacity is always better than too little.

A CO2 diffuser is a real asset for your aquarium. It’s easy to install yourself, looks great, and does an excellent job. You just need to maintain it from time to time. Beautiful and efficient—the perfect combination! 😉