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Why Do Aquarium Plants Need CO2 (and should you add it)

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When I started getting more into live aquarium plants in my aquarium, I kept reading about adding CO2 for better plant growth. I needed to know why plants even needed CO2, so I did my research and found the answer.

Aquatic plants combine water, light and CO2 into oxygen and glucose. This process is called photosynthesis and is the reason why plants need carbon dioxide. The CO2 concentration in aquariums is not enough for optimal plant growth, which is why people add it to their tanks.

Now that’s the short answer, but before you run to the store and buy expensive CO2 equipment, you need to understand that aquariums can be beautiful (with plants) even without additional CO2. The rest of this article will briefly explain what you need to know so you’ll have a perfect understanding of this topic!

Tip: I’ve composed a list of plants that can do great without additional CO2, which you can read here.

How much CO2 is there in an aquarium

It’s important to understand that your aquarium water already contains some dissolved Carbon Dioxide. This CO2 got into your water through gas exchange with the air at the water surface. If you’ve got a filter output or some other water current pump that’s disturbing the surface of your aquarium, oxygen and CO2 will enter the water.

In an average aquarium there will be between 3 to 5 parts per million carbon dioxide, which your plants can use to grow. If you’ve got slow growing plants and you keep them under low to moderate lighting, this amount of CO2 will suffice.

However, if you’re adding more light you encourage your plants to grow faster. At a higher rate of growth, your plants will ask more from your aquarium in terms of nutrients and carbon dioxide. At this point you may need to reduce your lighting or increase the amount of CO2 in your aquarium to ensure the health of your plants.

Why do we have to add CO2 to our aquariums

In nature, the fish that we are keeping swim in water which contains way more CO2. For the plants, all components needed to grow are readily available. The CO2 enters the water through the substrate (mud), decaying organic matter and other biological processes.

In our tank, all this is contained to a minimum. Sure, the respiration of our fish produces CO2, but this is such a small amount we don’t even take it into account.

Because of this, the concentration in our aquarium is way lower than what the plants need to grow. A nutrient deficiency could cause algae to grow. I’ve written a post on why plants can turn brown, which you can read here if you find it interesting. If we’re looking to recreate plant growth resembling the way they grow in the wild, we’ll have to add additional CO2.

Aquarium plants that do well without additional CO2

Without repeating myself too much, you by now probably understand that you can keep plants as long as you don’t want the to grow too rapidly. High light is necessary for some plants, so you’ll want to avoid those. Bright lighting also can cause a lot of algae. Part of that algae problem can be overcome by adding CO2, but let’s focus on not adding any for now.

1. Java Fern

If you’ve looked at plants already, you’ll for sure recognize a lot of these names. Java Fern plants are super popular, beautiful, large, lush green plants that are readily available and can be grown without additional CO2.

The plant does have roots, but it’s best not to bury them because they can start to rot. Know that this is an exception for this plant and the next one on the list, but other plants with roots are usually made for planting in substrate.

It’s best to attach these plants on a piece of rock, driftwood or even an ornament in your aquarium. To do this, use some fishing line or superglue.

2. Anubias

Anubias plants are sturdy plants that grow slowly. Because they grow slow, they don’t require a lot of fertilizer and added CO2, which makes them great for beginners. This plant is a rhizome plant, and grows roots that also should not be buried.

There are many different varieties, from smaller ones that are more suitable for smaller tanks, to pretty large ones. I’m certain that your local aquarium store has some of them in stock, so check them out.

They can be a bit more expensive, but they are beautiful and very hard to kill!

3. Cryptocoryne

Cryptocoryne plants, or “Crypts” for short, are popular in the hobby. They do great without additional CO2, and therefore deserve their spot on this list.

They are beautiful and easy to keep as well as easy to propagate. Check them out if you’re looking for some.

Cheapest DIY way to add CO2

Before it looks like I am trying to get you to buy a $200+ carbon dioxide setup (which I am not), you must know about the DIY way of adding CO2 to your aquarium.

To make a DIY Carbon Dioxide setup, you need a plastic soft-drink bottle, an airstone, some airline tubing and a return valve. This setup utilizes the reaction between sugar, yeast and baking soda to produce CO2. The tube will feed the gas from the bottle to your tank.

The first step is to cut a hole in the cap of the bottle, that tightly fits the airline tube. Seal it with (aquarium-grade) silicone so no gas can escape. The return valve must be positioned between the bottle and the airstone, preferably at the height of the water level.

This check valve makes sure no water can be sucked from your tank, and allows the gas through one way.

Use something like a suction cup to position the airstone inside your aquarium.

Finally, you’re going to fill half of the bottle with water that’s not too warm and not too cold. Add a cup of sugar and shake it up, to dissolve as much of it as possible. Now add more water to cool down the solution to a point where you can feel it’s still warm, but don’t leave it too hot because this can kill the process.

Finally add a teaspoon of bakers yeast and a pinch of baking soda. Close the cap and CO2 will start to form. Now double check that your return valve is positioned the right way because otherwise the pressure in the bottle will rise and you’ve just created something that might explode when you leave it unattended.

I encourage you to look up a detailed video tutorial (I am for sure going to make one in the future, but it’s not online at this point in time).

Advanced way to add CO2 to your tank

There are many kits out there that give you all you need to add CO2 to your aquarium. Here is a list of all the componenets:

  • CO2 tank
  • Regulator
  • Solenoid valve
  • Return valve
  • CO2 Diffuser
  • Drop checker
  • Bubble counter
  • CO2 proof tubing

Without going into much detail here, the regulator is attached to the tank and regulates how much CO2 is leaving the bottle at once. You use the bubble counter to check how much you’re adding by counting the amount of bubbles per second or per minute.

The solenoid valve is essential if you do not want to either spill CO2 or kill your fish. The valve allows us to stop adding CO2 at night, when the plants don’t need it. You can chuck the valve on a timer so we don’t have to turn it off every evening ourselves.

The return valve serves the same purpose as with the DIY CO2, and makes sure the tubing is not filled with water, siphoning your tank empty. Make sure you position this valve around water level.

The diffuser is a fancy way to let out tiny bubbles of CO2, increasing the amount of surface or prolonging the time the bubble is in contact with the aquarium water.

Finally there is CO2 proof tubing, which is essential if you don’t want to spill any CO2. You’ll receive this with your kit or you can buy it at your local fish store.

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