Do You Need A CO2 System For A Planted Aquarium? Get The Truth

Like all kinds of green plant species, aquarium plants also need CO2, and it’s a must-needed nutrient for them. So, do you need a co2 system for a planted aquarium? It’s a reasonable question asked by many aquarists often. But, unfortunately, the answer is not that straightforward and needs a bit of explanation.

Although aquarium plants require co2 to survive and grow, a co2 system is not always essential for planted tanks. Because co2 naturally occurs in the aquarium water, and it can be sufficient for your plants.

But in some cases, aquarium plants need an additional co2 system to grow and flourish. Primarily, it depends on the types of plants you have and some other factors. For example, if you want to have a densely planted tank with flourishing green plants, you might need a co2 system at some points.

In this article, I’ll talk in detail about plants’ co2 requirements to help you for a complete understanding of this matter.

Why Do Aquarium Plants Need CO2?

You’ve heard about photosynthesis, which is crucial for the growth and survival of green plants. Photosynthesis is a photochemical reaction where the plants use co2 to produce sugar and oxygen in the presence of light, which they utilize as a source of energy to grow and survive.

Photosynthesis takes place in the green leaves and stems of the plants. Light and carbon dioxide are basic needs for plants, and your aquarium plants are not exceptions.

do you need a co2 system for a planted aquarium_summary

Do I Need CO2 In My Aquarium?

No doubt, co2 is essential for aquarium plants to and flourish as it’s their biological need. The question is, do I need to add it additionally?

co2 does present in the water naturally. 3 to 5 ppm of co2 in the water is sufficient for most aquarium plants, and that level of co2 can naturally be present in a planted tank.

co2 enters the water from the atmosphere, from fish respiration, other microbiological processes. It is interesting to know; plants also release co2 by respiration. In photosynthesis, plants absorb co2 and light to produce sugar and oxygen.

Sugar and oxygen react to produce energy, water, and co2 in the respiration process. So, a portion of co2 actually is cycled by plants instead of being fully absorbed.

co2 and light both are essential for photosynthesis. So if you maintain low to moderate lighting with no high-light demanding plants in the aquarium, there is no need for co2 supplementation.

When A Planted Aquarium Requires a CO2 System?

The photosynthesis process is the primary source of energy for aquarium plants. And the process needs light and co2 at a balanced level. So if you provide high to moderate lighting for the plants without co2 addition, it will do no good for them but promote algae growth.

Sufficient co2 with adequate lighting can do magic to your aquarium plants. They will be healthier and flourish with a more vibrant look. Even though low-light plants don’t require a co2 system, they also do better with sufficient light and co2.

At the same time, you have to ensure the plants get macronutrients and micronutrients, like phosphate, potassium, magnesium, iron, etc. Because to get the full benefit of more light and more co2, the supply of nutrients must fit.

How Much CO2 For Planted Aquarium?

As I said before, the necessary co2 level solely depends upon the lighting requirement of the plant. Usually, three categories of aquarium plants are familiar, based on their light needs. I discussed each type with the definition and their co2 demand.

Low lighting plants (co2 requirement: 3 – 5 mg/l)

low co2 plants

Low lighting plants are typically those that require 1.5-watt lighting per gallon. This is a gross estimation. You have to consider full-spectrum fluorescent or LED lighting to get this measurement accurate. And this law is applicable for more than 10-gallon size tanks.

For low light-requiring plants, 3-5 mg/l co2 level is sufficient. This level of co2 is naturally attainable in planted tanks. I don’t suggest a co2 system for low-light plants. It required extra cost, time, and labor for maintenance.

Anubias, Java Fern, Java Moss, Marimo Ball, Dwarf Rotala are good examples of low light plants that you can grow in the aquarium nicely without a co2 system.

Medium lighting plants (CO2 requirement: 10 – 15 mg/l)

medium co2 plants

On a similar scale, a medium-light-requiring plant needs 2 watts for lighting per gallon.

For a modest growth of these kinds of plants, 10-15 mg/l co2 is ideal. They can grow without a co2 system but will thrive if you have one. If you have the budget and knowledge to operate a co2 system, I’ll advise you to have one. But for a small tank (less than 20-gallon), it is not so essential, I think.

Cryptocoryne, Amazon Sword, Vallisneria, Hornwort, Duckweed are medium light aquarium plants. With co2 and light, they grow faster with bright, larger leaves.

High lighting plants (CO2 requirement: 15 – 30 mg/l)

High-light-requiring plants are those that need more than 3 watts of power for lighting per gallon.

15-30 mg/l of co2 is the optimum level to keep these plants healthy and flourishing. Therefore, a co2 system is essential if you have these plants in the aquarium. Only intense lighting but limiting co2 will not give good results for these kinds of plants. Instead, it promotes algae bloom in your aquarium.

Monte Carlo, Dwarf Baby Tears, Staurogyne Repens, Ammannia Gracilis, Scarlet Temple are high-light demanding aquarium plants.

What Is a CO2 System for Aquariums?

aquarium co2 system diagram

A co2 system for fish tanks is the arrangement of supplying additional co2 to the aquarium water. The intention is to increase the dissolved co2 concentration of the water. A typical co2 system has five critical parts:

1) co2 tank:

It stores co2 gas to supply. It is known as the co2 bottle as well. It contains pressurized co2 at a higher pressure, and it can be up to 860 psi, depending on the bottle type.

2) co2 regulator:

It regulates the amount of adding co2. The regulator steps down the co2 pressure from the bottle pressure to lower supply pressure. There are different types of aquarium co2 regulators are available in the market. co2 regulator is an important item of the co2 system as it regulates the supply amount.

3) co2 diffuser:

CO2 diffuser forms small bubbles to ensure uniform distribution. co2 passes through a porous medium and forms mist-type bubbles. Tiny bubbles are easily absorbed with the aquarium water. 

4) Solenoid:

A solenoid is an automatic shut-off valve that you can interlock with a preset timer or logic. For example, you can keep the co2 system off at night during the light-off period with an automated system consisting of a solenoid. Thus it will save wastage of co2.  

5) Bubble counter:

A bubble counter allows monitoring of the rate of injecting co2. You can count the bubble of co2 entering the water per minute. It will give you an easy indication of the volume of co2 dosing. 

Apart from these five components, a complete co2 system may have other parts like pressure gauge, tubing, drop checker, etc.

Signs of Too Much CO2 In Aquariums

co2 is essential for live plants in the aquarium, but it is not friendly with your aquarium fish. If the co2 level in the aquarium water is too high, it can be fatal for fish and invertebrates. The safe level of co2 in aquarium water is below 32 mg/l (ppm).

If the co2 level is higher than the safe limit, aquarium fish will suffer and show the following symptoms:

  • Grasping in the air
  • Loss of appetite
  • Lack and movement and lethargy

If you observe this sort of symptoms, especially in a tank having a co2 system, check the co2 level immediately and perform a water change as quickly as possible. Check our detailed article on signs of too much CO2 in an aquarium. 

How To Set Up Aquarium CO2 System

Get a video tutorial:


co2 system is not for beginners. It requires extra time, effort, and cost to maintain appropriately. If you are a newbie and start a planted tank, I’ll suggest you choose plants that are happy with less food. That means they are low-light-demanding, and so natural co2 will be OK for them.

Basically, a co2 system is for them whose primary focus is not only fish but also different aquarium plants. You can still somewhat maintain a planted tank without a co2 system, which is not huge in size and not densely packed with ambitious plants.

If you decide to install a co2 system for the aquarium, ensure you have a safe and reliable package. If you are not an expert on this matter, follow the manufacturer’s instructions to the letter when setting the equipment.

Sujit Modak

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