CO2 generator – a DIY for a tropical aquarium

My 110L aquarium

I decided some time ago to take my tropical aquarium a planted tank.  They’re just better. So I need a CO2 Generator.

Whats needed for a planted tank?

I had all the basics of a good aquarium already – Tank, water, fish, heater, filter, lights, substrate, etc. but it seems it wasn’t enough.  So i did some reading on what i might need to do it and do it properly – so I made a list of what I needed…

A CO2 Generator helps fish too!
shutterbusterbob / Foter / CC BY-NC-SA

  • More lighting (Few W/Gallon)
  • Suitable substrate
  • Plants
  • CO2 injection system
    • CO2 generator
    • Bubble Counter
    • Diffuser
    • Tubing
    • Solenoid
    • Pressure Gauge
    • Reagents
  • Drop checker
  • Fertiliser

It was at this point I looked at how much it would cost to get this ‘off the shelf’.  Then I swore loudly.

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.

Why a 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?Recipes for a 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 much longer time – which makes it better to use if you don’t have a regulator.

Basically, it’s brewing.  So if you like you can inject CO2 into your aquarium and make beer or mead with no real issues.


  • 20g baking soda
  • 400ml 5% vinegar

This will make a LOT of gas and bubbles quickly, so make sure your reactor can take the pressure!  Simply add the baking soda to the reactor, then pour in the vinegar quickly and seal as fast as you can.  Use a clamp or needle valve on the gas line to control the flow rate.


  • 250g sugar
  • 300ml lukewarm water
  • 2.3g dried yeast

Take a little sugar and water and use it to activate the yeast for at least 10 mins (a tablespoon works well for this).  Add the remaining sugar and water to your reactor (don’t shake it!) and then add the activated yeast and seal the reactor.  This should produce CO2 within the hour and make a steady flow for around 7-10 days when in a 2L bottle. Again, control the flow rate with a needle vale or clamp on the gas line going to the tank.

These recipes will need adjusting to your personal aquarium.  You may find more or less of any of the ingredients will work better.  Not helpful, but it’s a fact I’m afraid.

Make a CO2 generator system for less than £15

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:

  • 3x 2L Soft drink (soda) bottles – Save these up over a few weeks (free, sort of)
  • 1x 4mm drill – Had a drill already
  • 3x Check Valves – £3 from your local fish shop or amazon
  • 6mm airline tubing – 1m + enough to get from the gas separator to inside your aquarium (I used 2.5m in total) – about £2-£3 at your LFS
  • Aquarium safe silicone sealant – £6 from B&Q – use only 100% silicone sealer, free from additives like anti mould.  (I would skip this and just use brass nipples with nuts to secure, you get a better seal this way)
  • Sugar
  • Yeast
  • Water
  • Airstone

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

Putting it together
What you need to do is drill 1 hole in 2 of the caps, and 3 holes in 1 cap.

With the single holed caps, you add some airline and a one way valve that allows gas to go from the yeast mixture out of the bottle.

With the triple holed cap, you want to attach air tubing to all 3 holes -2 of the lines should be from the 1 holed caps made earlier, and the other should be long and have a one way valve on it.  The line with the one way valve is the one that goes to your aquarium, so attach the stone on the end.

This cap is for the smaller bottle – it allows it to act as a CO2 store for when your changing bottles.  It can also catch some foam coming from the CO2 generators if you get the mixture wrong!

I originally pushed the tubing through the holes in the caps and siliconed them in place (as the photos) but I’ve since replaced them with brass nipples that can be pushed through the holes and bolted in place from the inside – it gives a much better seal than silicone.  Especially as the silicone doesn’t seem to stick for long.

Fine tuning
You can attach an in-line valve in the line coming from the smaller bottle to control the gas flow into your tank, but you’ll need to play with the recipe if the maximum flow isn’t enough or too much (usually the yeast amount)