A better CO2 generator for £25 – or £35 if you’re fancy.

I decided some time ago to make my tropical aquarium a planted tank.  They’re just better. So I needed a better CO2 Generator. Now I need a better one of those as it turns out it’s tricky to get a long-lasting, gas tight seal on the cheap.

What’s went wrong with the other set up?

It seems my previous attempt was pretty danged good and worked faithfully for several months with no real issues, but as its aged a few problems had cropped up:

  • Leaks around the cap to CO2 line joints.
  • The brass nipples were prone to corrosion and one snapped off.
  • Dissolving CO2 into the aquarium water was tricky to get right.
  • Difficult to keep a constant flow rate (bubble count).
  • Large amounts of organic waste that needed disposing of.
  • Couldn’t stop CO2 generation overnight and so was wasting gas for no good reason.

Clearly something needed to be done.

So I decided to make my own as cheaply as possible!  In this post, I’ll be looking at the CO2 system – specifically the CO2 generator. The solenoid and pressure gauge are just getting fancy so that’s a while away from being done yet.

How is it a better CO2 Generator?

Oh, and It’s CO2, not CO2.

Why are CO2 generators needed?What do CO2 generators do?How do CO2 generators do it?Recipe for this CO2 generator

Plants make their own food. It’s true – it’s even why they’re green!

Plants make the food they need to survive through photosynthesis, using light, water and CO2. Aquatic plants use exactly the same process as plants above the water line, so they need the same things.

The only real difference is that aquatic plants absorb CO2 dissolved in the aquarium’s water instead of directly from the air like their land-based cousins.

A CO2 generator is a device that makes CO2 gas.

It is as simple as that.

This can be done in a few ways which can be divided into 2 broad categories – chemically and biologically.

Chemically based generation is making CO2 by mixing 2 chemicals together like baking soda (sodium bicarbonate) and vinegar (acetic acid) which give off a lot of CO2 very quickly.

Biological generators use water, sugar and yeast to make CO2.  This tends to make a similar amount of CO2, but over a longer time – which makes it better to use if you don’t have a regulator.

It seems doing things chemically is easier, less messy and more reliable.


  • 200g Citric Acid & 400ml water – Bottle A (for acid)
  • 200g Baking Soda (Sodium Bicarbonate) & 200ml water – Bottle B (for Base/Bicarbonate)

Make a better CO2 generator system for less than £25

The build

The concept
The idea behind a CO2 generator is so you can have a system that easily and cheaply produces (generates) a steady supply of Carbon Dioxide to ‘feed’ your plants.  As I have a large-ish aquarium, I want to have a system where I can have two generators – this means I can change one of them every week and have lots of CO2 with no major dramas or more than 10 minutes extra maintenance.

I’ll also be using a gas separator – a bottle the generators feed gas into that then pipes it into the aquarium.  This is just in case something goes wrong and to ensure no yeasty mixtures end up in the aquarium.

What's needed
For my system, I am using:

  • Professional DIY CO2 Generator Check Valve Kit Set For Planted Aquarium D501
  • 2x 2L Soft drink (soda) bottles – Save these up over a few weeks (free, sort of)
  • 1x Check Valve
  • 6mm airline tubing – 1m + enough to get from the gas separator to inside your aquarium (I used 1.5m in total) – about £2-£3 at your LFS
  • Water
  • Bicarbonate of Soda (Sodium Bicarbonate)
  • Citric acid
  • Airstone

All easy things to get hold of and should total less than £15 for everything.

Putting it together
There are two mixtures which need to be prepared accurately, one in each 2L soda bottle.

  • For bottle A, use the Citric Acid mixture detailed above in the “Recipe for this CO2 generator” section.
  • For bottle B, use the Bicarbonate of Soda mixture detailed above

They are then twisted into “the device” properly (Bottle A has the 3 way connector init and the bottles are screwed in firmly) , and once you make sure the needle valve is closed, bottle A is squeezed so a fair bit of the liquid goes into bottle B.  This produces a lot of CO2 gas quickly, and we want to get the bottles up to around 1-2 kg cm3 of pressure so the needle of the gauge is in the green and remain stable.

Fine tuning
To fine tune the CO2 system there are a few things to consider:

  • The health of your fish!  Test the water regularly with test strips or a liquid test kit.
  • Use a drop checker – This helps you measure the CO2 concentration in the water – typically 20-30ppm of CO2 makes the liquid in these turn light green and is ideal for most plants and safe for wildlife..
    • A the drop checker is effectively measuring the pH of the Carbonic acid vapour exchanged between the tank and the air in the drop checker there is a 2-3 hour delay in readings.  This means any changes won’t be detectable until a few hours after they were made.
  • Using a needle valve (included in the set I used) lets you finely control the rate at which CO2 is released into the aquarium.
  • Using a bubble counter s an effective way of measuring the amount of gas going into the aquarium – I find 1 bubble a second is about right for my tank.

Increasing Carbon Dioxide levels in the water

After all, there’s no point in making a better CO2 generator if you can’t actually get the CO2 into the water!

If you are using an airstone the CO2 will leave the aquarium much faster as the extra surface agitation of the water will increase the rate of gas exchange and the flow of gas is always from high to low concentration i.e. the high concentration of CO2 you want in your tank is literally evaporating into the atmosphere and that’s a waste of gas.

To dissolve more CO2 into the water there are several methods that have ok to excellent results:

  • CO2 reactor
    • A device which connects to an external filter’s output on the way back to the aquarium where you pump in CO2 and it’s mixed with the running water directly.  Very effective and can take up some space.
  • Ceramic diffuser
    • Usually made from glass tubing with a flat ceramic disk though which the CO2 is pushed (by higher pressure) and made to form very small bubbles.  These small bubbles dissolve into the water much faster than the larger bubbles produced by a pipe and/or an airstone.
    • I got this one:
      from here for around a tenner – this is the fancy part of the setup..
  • Pumping CO2 directly into the filter inlet
    • Works a treat!  Just by piping the gas into the inlet of an external canister filter you can achieve up to 100% of your gas dissolving. Simple, effective and cheap (if you already have an external filter).

I found mixing the last two methods works well and doesn’t cost much.