Step 1: Materials

Picture of Materials
You will need:

A one liter gatorade bottle (or similar)
Several feet of airline tubing (vinyl is ok, but silicon is better)
An air diffuser
A suction cup to fit your tubing
A check valve
Aquarium grade silicon
A planted tank (duh?)

Step 2: STEP ONE

You want to make a hole in your bottle cap that is just a wee bit smaller than your airline tubings overall diameter. I used a pair of scissors to carve a hole, simply for the fact that I was too lazy to get out the drill, find the appropriate bit, and put it away, just for one hole. If you have more gumption than I, by all means, go ahead and drill it.

Step 3: STEP TWO

Make a slanted cut in the end of your airline tubing. This will make it easier to get in the hole on the cap.


Pull your airline tubing about 1.5" through the cap. If you are having troubles, try pulling with a pair of pliers. If you are STILL having trouble, you can shave a bit from the hole with a pair of scissors, or use a larger drill bit. Your call.


Ok, this is the part where I messed up when I was taking the pictures. Bear with me.

About 6-10 inches away from the cap, cut the line in half, and plumb in your check valve.

MAKE SURE that the arrow is pointing AWAY from the bottle cap, otherwise you will have a ticking yeast bomb sitting under your aquarium.

The picture is of the finished project. Just pretend you don't see that part though ; )
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so i did this and it seems to be producing just fine.. i placed my air stone about an inch under water in my 55gal aquarium. Will this be deep enough to work? i also dont see any bubbles coming from my tiny air stone. what is going wrong? i have everything completely sealed off (air tight) with a very thick bond of glue. Whn i shake the bottle it bubbles like crazy and thats the only time i see the air stone pumping co2 out. HELP please, my plants have been doing

place your airstone little close to water level... sometimes to due less pressure it does nt come when it go down

MarkS252 months ago

You may better control the output from these by putting the entire contraption in a tightly sealed (tall kitchen) plastic bag with an air pump. With the pump on, CO2 is delivered to the tank by the pump - pump off CO2 inflates the bag for use later. Also, 2 bottles, started and replaced about a week apart make for a more consistent supply.

Can i leave my outside filter on during the day with the CO2 system. & pull the tubing out of the tank at night to keep the fish safe? Would this work?
jules152 years ago
Can you please tell me what you're using for substrate? and lighting? do you use fertilizer? Sorry if you already answered these questions previously, but your tank looks BEAUTIFUL and i want mine to look like that. thanks in advance :D
ksprabu2 years ago
or u can use soap solution for smoothing out silicon 
piesforyou5 years ago
Could you go into more detail about why the baking soda is needed? How does it "keep it steady"?
one of the byproducts of sugar fermentation by yeast is alcohol.  with the diy co2 reactor sealed, there is no way the alcohol escapes so it remains inside the container.  the alcohol alters the water's chemistry by changing the pH which, at some point kills the yeast.  the more yeast you add, the faster co2 (and yes, alcohol too) is produced.  the baking soda is added to buffer the change in pH and make the mixture produce yeast longer. 

i used the ff. recipe myself (using a 2-liter softdrink bottle)
2 cups sugar
1.5 liters water
1 tsp baking soda
0.25 tsp yeast

prior to using baking soda, my mixture at best lasts for 10 days.  after i added the baking soda, the mixture lasts at least 3 weeks.  the only problem i have with diy co2 is that the co2 bubble rate cannot be controlled precisely.
You could add one of those little flow rate "taps" inline before the check valve, as long as it's never closed all the way (that could get messy) -- this is what I mean
Mitten6 years ago
Silicon =/= Silicone!!!!

That is my BIGGEST pet peeve.

Silicon is the metalloid used in semiconductors.

Silicone is the caulking and breast implants.

And now you know!
Sgt.Waffles (author)  Mitten6 years ago
Maybe you would get more out of an instructable if you didn't fret about trivial things such as an "e" or lack thereof. But thanks for the heads up.
. Maybe you would get more out of constructive criticism if you weren't so sensitive. ;) There is a BIG difference between silicon and silicone.
such as a typo?
Sgt.Waffles (author)  NachoMahma6 years ago
Minor details such as that don't really matter in the long run. You got the point I tried to get across, no?
oitsthatguy3 years ago
Awesome instructable! For the air diffuser, might a standard aquarium air-stone work?
AmyLuthien3 years ago
Or you can just lick your finger when you're smoothing out silicon. That's what I learned from my old dad years and years ago! ;)
Tracx4 years ago
Hi there Sgt. Waffles,
Even a long time after your tutorial, I've found it on the web and found it very useful. I've managed to get all the required equipment including a fancy mounting-thingy for my Nano cube for less than 10 euro's.

The CO2 exits just below the filter, where it gets dissolved in the water really quick. At night, when the filter is off, the excess co2 bubbles up and escapes at the surface.

I've seen bizarre results:
I was looking for a way to better grow my plants as they were getting brown and ugly.
I found CO2 was the solution, thanks to your tutorial a cheap one too, and now have nearly 100% green (some brown left) plants, that are happily growing -- some even centimeters per day.

Big thanks!
There is a very dangerous practice of letting this reactor go all the time.It should only be run while the lights are on.Plants reverse their co2 intake at night and this can very well kill all your fish. I would only run this every 2 to 3 days.I plan on making one thanks for the ingenuity and time.
EdgarVanKat5 years ago
Could I use backing powder instead of backing soda or would that have a different outcome?
This is a late comment, but those two things are very different chemicals.
kjfan15 years ago
toddstivers5 years ago
ive tried this and it doesnt work i need help
ace0075 years ago
Just get the bit out and turn it with your hand :-) If it's still new it will give you a perfect hole for the tubing
rklee20205 years ago
hi there it sounds like it will work great & very inexpensive, just one question though. is there a better time to have it runnning, eg day time only or night time only or can u leave it on 24 hrs. when i first started using this, my fish at night would all be at the surface acting like they where gasping for oxygen. so i turn it off during the night & only turn it on for a few hours during the day. & another question when do plants take in the most c02, daytime or night time? so i will be able to turn it on when its the most beneficial for my plants.
the ideal scenario would be a co2 reactor that runs only when there is light as that is the only time plants photosynthesize and give off oxygen as a by-product.
at night when there are no lights, plants use oxygen and give off carbon dioxide.
this can only be accomplished with pressurized co2 setups that use solenoid valves and a timer switch.

for diy co2 reactors, they are usually left running 24/7.  to avoid too much co2 buildup (e.g. at night), just make sure there is water movement at the water's surface.  when the lights are on, keep surface water movement at minimum when you have a planted tank.
acosicris5 years ago
good instructable..

i have a question, in step 4 what do u call that part of airline with arrow? Or should i ask.. where can i get similar of that? (in medical dextrose?)

Shaybah5 years ago
Super Idea! Leave the lights on 7/24, but try to position the the lighting so it doesn't shine on the front glass. The green stuff on the glass is not pretty. Another idea, you can use real fruit juice, and cut back on the sugar. I didn't see anyone asking what to do with the Solution left in the bottle after the bubbles stop. I'm just a country boy. Best Regards
do you need the check valve.
smegman5 years ago
Is that dwarf sag? yours is beautiful. can you give me any advice on lighting and substrate?
KDSinLA4446 years ago
I built a CO2 reactor very much like this one about a year ago, hooked it up to my (very small, 2.5 gal) tank, watched the little bubbles form abd float around, went to bed, and in the morning three of my fish were dead. I immediately disconnected it. And then started to wonder what was going on... ...Obviously, as this biological CO2 reaction is going on, carbon dioxide is being produced... But is not the only thing being produced! I understand that such reactors seem to work quite well for other aquarist, but when I took a whiff of just what WAS being pumped into my aquarium what I smelled was ALCOHOL! Now, of course, the brew is supposed to produce alcohol... But alcohol is also extremely toxic, and I can't imagine that some of it wasn't evaporating out of my CO2 reactor and then getting pumped into my little tank along with the desirable CO2. I have read several articles about these home-made CO2 reactors but I have never seen anyone address the issue of the possible absorption of the highly-volatile alchol into aquarium water. And I don't understand why... I mean, the stuff stinks! Who wants to pump "stinky" gas into their aquarium??? Also: couldn't I just hook up a bottle of carbonated water to my tank once a week or so and let the released CO2 from that provide me with all the carbon dioxide I need? And wouldn't this be safer than risking alcohol poisoning my fish? I am currently considering trying to used compressed CO2 from a local paint gun store, but I do not have the money to spend on regulators and bubble counters. I need a cheap, effective, and NON TOXIC method to aid my plant growth in a planted and fish-occupied MINI tank-- and I really don't want any more dead fish... Ideas??? Thanks!
Clearly, your Co2 setup produced way too much CO2 for your tiny tank. These setups are generally sized for 10g-20g tanks. You could try doing a little more research to find out if there is a formula for sizing to your tiny tank. Also keep in mind that plants produce CO2 in the absence of light. I.e. at night.
noidea6 years ago
What size tank is most suited for? Would this be ok for my 180 litre tank? And what size is the air line? i have lots of 6mm....
Here is one version you may like to try.
Take the bottle and fill to near the top with water. In the tank place a small plastic container with the open end pointing downwards. Insert airhose from bottle to the inside top of the container.

Go out and buy the cheapest Denture Cleaning tablets you can find. I find these are 30p in the UK for about 20.

Place one Denture cleaning tablet into the bottle with water. The gas given off is CO2 and is collected in the open-ended container in the aquarium. As the water circulates inside the tank, the CO2 dissolves over several days.

Once every month replace the water in the bottle as it becomes saturated with the chemicals from the tablet.

Note: it is important not to have too much aeration or surface water disturbance during 'lights on' as CO2 is very easily removed by this action. Have it on at night though as plants generate their own CO2 at night.

Note: you'll get better results if you add some cheap plant food.

(Poor Mans Dosage Drops)

This method I use very successfully in a 20L tank at my work for 2 years now. It's fun hunting down suitable containers in a supermarket. Try the kiddies section. They tend to have tonnes of useful small containers.

This is a posting from The Aquarium Wiki Encyclopaedia.

Dear Sir Could inform me the ingredients indicated in the package of the tablets , i will try to find a similar one to test it. Why this solution is not sugested by other, probably people does not know. regards
tcassaday6 years ago
I'd like to know what the overal price is for this set up
Tuttleo6 years ago
Neat! I've just set up the contraption to your specs for my new, planted 20 gal--It seems to be working--I think, or at least the check valve and expansion of gasses is working, since it's bubbling into the aquarium. I guess if the fish start gasping for air at the surface, I'll know it works a little too well. Is there some sort of test I can do to make sure that my CO2 levels don't exceed 30ppm? Do smaller tanks need smaller reactors? Also, how often will I need to mix up a new sugar water/yeast batch?
Sgt.Waffles (author) 6 years ago
Can you put that in a coherant sentence please? It doesn't need to be silicon tubing. Regular tubing works just fine.
davart6 years ago
Air tubing in to be CO2 grade, really aquarium tubing will harden and crack.
Sgt.Waffles (author) 6 years ago
I do what I can ;)
Guppymom6 years ago
Thanks a bunch for the inexpensive CO2 contraption. You just saved me at least $130 & my plants look great! You're the best!
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