Carbon in the planted aquarium: Gas vs. liquid


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George Farmer compares gas CO2 injection in the planted tank with liquid carbon fertilisers.

One of the easiest ways to achieve faster plant growth without having to supplement tank lighting is to add an additional source of carbon to your planted aquarium. Carbon is the most important nutrient for all plant life and in the last few years many more products are available for adding this essential macronutrient to the aquarium.

Do you need to supplement carbon?

Before choosing and buying your carbon supplementation, decide on whether you actually need it. The main factors influencing your decision are how much light do you have, what plants you wish to grow, how much maintenance you are prepared to undertake, and what budget you have.

CO2 injection/liquid carbon is essential in aquariums with high lighting levels. Without the extra carbon to feed the plants’ nutrient requirements algae will proliferate.  

High lighting levels are regarded as anything over 0.5 watts per litre/2w per gal, assuming you have T5 lighting with reflectors in a regular-shaped aquarium.

In nano aquariums (less that 40 l /10 gal.) high lighting is around one watt per litre. You will need high lighting levels to grow demanding plants, especially carpeting species where the light levels drop off considerably towards the bottom of the water column. 

The combination of high lighting and CO2 injection will result in super-fast plant growth and therefore more maintenance via regular pruning and water changes.

Conversely, in aquariums with lower levels of lighting and easy plants, carbon supplementation is not necessary, as there is enough naturally present in the aquarium water and substrate.  However, it can be supplied to help provide healthy plant growth and avoid potential algae issues.

Gas or liquid?

Carbon in its gaseous form comes from carbon dioxide (CO2) and can be supplied by three main methods in the aquarium; yeast-based, aerosol and pressurised systems.

Yeast-based systems rely on the byproduct from fermentation and provide a relatively inconsistent and uncontrollable supply.  

Pressurised systems are more reliable and controllable, but expensive. Aerosols are good value but only suitable for small tanks with little control over diffusion.  

Liquid carbon fertilisers have been around for a few years with more brands being released.  They all have the same active ingredient with similar dosing instructions, some being more concentrated than others.

Compared with gas CO2 they do not provide carbon as efficiently so plant growth is slower.  

However, they can work out better value, especially in smaller aquariums and have a positive side effect due to their adverse effect on algae.

CO2 is toxic if overdosed. Some fish are more sensitive than others, especially if oxygen levels are low, such as at night where plants respire taking in oxygen and producing CO2.  

A safe level is under 30ppm and in most circumstances that can be measured with a drop checker, 4 KH water and bromo blue pH reagent solution.

Tips when using liquid carbon

  • Carefully dispense your liquid carbon from the bottle into a pump dispensing bottle.  
  • Measure each pump; the best quality bottles dispense 1ml per pump. This allows you to quickly and accurately dose your regular daily amount.  
  • Buying bigger bottles of liquid carbon is better value than thinking small.  
  • Dose your liquid carbon at the same time every day — ideally just before lights on.
  • Liquid carbon is great on black brush/beard algae. Expose the affected areas to the air then paint it on with a small paintbrush.
  • Don’t overdose, as it becomes toxic in higher quantities.

Tips when using pressurised CO2

  • Buy the best system you can afford. Get a high quality regulator in particular and it should last a lifetime.
  • Off-the-shelf systems are ideal for beginners but work out more expensive than customising, like using CO2 fire extinguishers.
  • Some diffusers are ugly and bulky, so consider buying inline or glass diffusers.
  • Use a CO2 drop checker to ensure safe and effective levels.
  • Shut off the CO2 supply at night, using a solenoid to save on it.
  • Clean the glass/ceramic diffusers regularly in bleach to maintain their efficiency.


Do not confuse activated carbon used in chemical filter media with carbon, the nutrient. Carbon filter media has no nutrient properties for plants and can actually remove some nutrients.

Pressurised CO2


  • Faster plant growth   
  • Can be fully automated   
  • Essential for some plants   
  • Stable CO2 production   
  • Easily controllable   
  • Customisable for any size set- up  
  • Essential for very high energy set-ups


  • Toxic when overdosed
  • Expensive initial set-up cost
  • Daunting for beginners
  • Potentially dangerous high pressures
  • Fluctuating or poor levels cause algae
  • Re-fills can be expensive

Yeast-based CO2


  • Inexpensive to set up   
  • Inexpensive to run   
  • Simply to use   
  • Difficult to overdose   


  • Uncontrollable
  • Inconsistent
  • Not suitable for large aquariums
  • Requires regular mixture changes

Aerosol CO2


  • Inexpensive to set up   
  • Simple to use   
  • Difficult to overdose   
  • Ideal for small tanks    


  • Uncontrollable
  • Not suitable for large aquariums
  • Refills required regularly

Liquid carbon


  • Simple to dose   
  • Helps prevent and treat algae   
  • Great value in small aquariums  
  • No equipment in tank


  • Toxic if overdosed
  • Expensive running costs in large aquaria
  • Harmful to some plants

Costing Example 1:

125 l/27.5 gal planted aquarium

Let’s compare the typical setting up and running costs of a 125 litre planted aquarium with an off-the-shelf 500g pressurised CO2 system vs. liquid carbon.

Set-up cost:

JBL ProFlora CO2 Set M601 - £169.99

500ml Easy-Life EasyCarbo - £10.99

Running costs – CO2:

Assuming 500g lasts eight weeks with no shut-off solenoid  

Assuming 500g refill = £12.50

Annual refill = (52 weeks @ 8)* £12.50 = £81.25 per annum

Running costs – Liquid carbon:

Daily dose (1ml Easycarbo per 25 litres aquarium water) = 5ml/day

500ml bottle = 100 days

Annual cost = (365 days @ 100)* £10.99 = £40.11 per annum


CO2 initial year running costs - £251.54

Subsequent running costs per annum - £81.25

Liquid carbon running costs - £40.11

Costing Example 2:

600 l/132 gal planted aquarium

Now let’s compare the typical setting up and running costs of a 600 litre planted aquarium with a customized 5Kg pressurised CO2 system vs. liquid carbon.

Set-up cost:

Customised pressurised CO2 kit with solenoid and 5Kg CO2 fire extinguisher - £150

4 litres Seachem Flourish Excel - £75

Running costs – CO2:

Assuming 5Kg lasts three months with solenoid

Assuming 5Kg refill = £25

Annual refill = (12 months @ 3)* £25 =  £100 per annum

Running costs – Liquid carbon:

Daily dose (5ml Seachem Flourish Excel per 200 litres aquarium water) = 15ml/day

4 litre bottle = 267 days

Annual cost = (365 days @ 267)* £75 = £102.52 per annum


CO2 initial year running costs - £250

Subsequent running costs per annum - £100

Liquid carbon running costs - £102.52

Costing Example 3:

25 l/5.5 gal nano planted aquarium

Let’s compare the typical setting up and running costs of a 25 litre planted nano tank with nano pressurised CO2 kit vs. liquid carbon.

Set-up cost:

Colombo Flora-Grow CO2 Nano Set (95g disposable cylinder) - £74.99

250ml Easy-Life EasyCarbo - £5.99

Running costs – CO2:

Assuming 95g lasts six weeks

Assuming 95g disposable cylinder = £11.99

Annual costs = (52 weeks @ 6)* £11.99 = £103.91 per annum

Running costs – Liquid carbon:

Daily dose – 1ml

250ml bottle – 250 days

Annual cost = (365 days @ 250)* £5.99 = £8.74 per annum


CO2 initial year running costs - £178.90

Subsequent running costs - £103.91

Liquid carbon running costs - £8.74

So how do they both compare?

Comparing gas CO2 to liquid carbon objectively is not easy given the large number of permutations available, including CO2 consumption, CO2 system sizes and specs, and liquid bottle sizes.

I have simplified matters here by using three examples. left, ranging from a 25 litre nano aquarium up to a 600 litre tank.  The carbon supplementation systems have been selected to appropriately match the tank sizes.

The results would be very different if, for instance, we decided to run a customised 5Kg CO2 system on a 25 litre nano tank, or dosed a 600 litre using a 250ml liquid carbon bottle. Also note the difference in cost between customising your own pressurised kit with a large fire extinguisher vs. off-the-shelf systems.

No matter, from these examples we can see that liquid carbon is a viable alternative to a pressurised CO2 system from a cost perspective in all set-ups, even the largest. However, gas injection has distinct advantages over liquid carbon. It can be fully automated, and using a solenoid can make further savings.  

Relying purely on liquid carbon is not suitable for some plants, such as Vallisneria, and it cannot supply sufficient carbon to meet the requirements of very high energy set-ups. If you want fast growth there’s no better way of adding carbon than via CO2.  For some the ultimate is to see their tanks full of oxygen bubbles, or pearling, a result of saturating the water with oxygen via photosynthesis. For this, there’s no better alternative to a pressurised CO2 system.

However, you can’t deny the cost effectiveness of supplying extra carbon via a liquid product in smaller tanks, especially when you consider in the first year our nano example costs more than £175 for gas compared with under £9 for the liquid alternative.  


One of the easiest ways to keep a planted tank algae free is to use relatively low lighting, such as enough to grow undemanding plants with liquid carbon and a comprehensive liquid fertiliser.

The anti-algae properties of liquid carbon products double up with the low lighting to help prevent algae. The liquid fertilisers ensure the plants are sufficiently fed to prevent nutrient deficiencies.

Did you know?

Around 40% of a plant is carbon. This compares with almost 20% in the human body.