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Last Updated 5/30/2014

Okay... so you have been wondering how people are making
their own CO2 reactors / generators to promote aquatic plant health and growth...
Well, here it is!

The Picture Guide to DIY CO2
(I am happy to share this with everyone, but would love to get a little credit if you re-produce this document, and would prefer you link to it instead.)


Keep in mind the fact that there are 3 major variables in plant health and growth. Light, Food, and Air (CO2). Any one of these not being available in the right perportion can favor the more simple plant life of algae instead of your desired aquatic plants. It is pretty easy to add plant food (mostly present from fish and fish food wastes). Similarly, you can add more lighting to your tank by changing out the hood / lighting. And even more simply than changing out your lighting, is adding a little CO2. However, while science is hugely involved, it is more of an art to get the right ammounts of all three, since you are the only one who can deside how you want your plants to grow. With that in mind, I suggest you 'play' with the three variables slowly and gently until your tank and plant growth are what YOU want.

The following is my simple picture guide of How to Make a Do It Yourself CO2 Reactor.

(Remember, you may want to make two to keep a more constant flow of CO2 for your plants... Wide fluctuations in CO2 favor the growth of algae. I use two 64oz Juice Bottle set-ups for my 55g that has just under 2 watts per gallon of light.)

Collect your matterials.

  • bottle (I use one liter soda bottles with the big caps)
    I later found I like the large square 64oz Juice Bottles if you have room for them. - They have a more stable base than soda bottles. The proccess for making this type of bottle is the same as below.
  • length of air tubing
  • air line check valve
  • straight air line connector
    note: some people just use airline pulled through a smaller hole in the bottle lid... while this works for many, I have never gotten a perfect seal this way and prefer the connector
  • drill with bit just a tiny smaller than your straight line connector
  • sissors
  • plyers
  • diffusor

Drill a hole in the cap that is just a tiny bit smaller than your line connector.

Use the plyers to gently push your line connector through the hole.

Attach your air line to the line connector and push it right up to the bottle cap.

Measure your air line and cut where it will be just above the top rim of the tank.

Attach your flow check valve with the arrow pointing away from the bottle.

Attach a diffusor to the end (sometimes I tie the diffusor to a suction cup to hold it in the correct place when it is in the tank).

You are ready to fill your bottle!

For a one liter, I use the following mix: (double for a two liter)

  • one cup sugar
  • one teaspoon baking soda (to help slow and regulate the reaction)
  • one-quarter teaspoon baking yeast (champagne or other alcohol yeasts will last longer)
  • hot and cold water up to the narrowing point of the bottle

First, use a funnel to add your sugar and baking soda.
Then fill about half way with HOT water and swirl to desolve your sugar.

When you sugar has fully disolved, fill the rest of the bottle to the point it begins to narrow with cold water. The resulting water should be about room temperature. If it is not, let it cool down before continuing.
Then add your yeast and gently swirl until the yeast is mixed into the water.

Store your extra yeast in a cool and dry place (I put mine on my fridge).

Put on your cap and you are ready to go.

Place your bottle in a location that it will NOT be tipped over. If there is any worry of tippage, you may want to install a bubble-counter/trap to keep the yeast mix from pumping into your tank. (will show at end of tutorial below)

Depending on your house temperature, yeast virility, and other factors, it may take as much as several hours, or as little as a dozen minutes for your CO2 to get going. Do not be concerned if water goes several inches up your air line from the inside of your tank at first. This usually is because your mix was a little warm and is cooling, drawing water temporarily up the tube from your fish tank.

If you do not see bubbles within 24 hours, you may need to rinse out your mix and re-do it. The most common reasons for not bubbling is having dead yeast, having your water too warm when the yeast was added, or if you do not have tight seals at your connections. If you suspect a connection leak at the bottle, place the bottle under water and see if any air is leaking at the seal. If you have leakage at the straight line connector, you can pull the connector out and put a little super glue around the piece before putting it back in.

WARNING!!! You can over-dose your fish by adding too much CO2 without enough surface movement. I can not stress enough that you should have good surface movement to out-gas your CO2 when you first add it, and then SLOWLY reduce the surface movement over several days to get a good balance of CO2 in your water. If your fish start to act funny, gasp at the surface, or lay still on the bottom, remove the CO2 and increase surface movment. Then you can start again and go slower.


  • The best tip is to use TWO reactors and start them about 2 weeks apart. (Or a month apart for larger reactors.) This allows for a more steady rate of CO2, since one bottle is getting going while the other is running out. Dramatic fluctuations in CO2 favors the growth of algae.

  • Increased CO2 in water causes the pH of your water to drop. This allows you to measure the amount of CO2 in the water by measuring pH in relation to KH. This requires a kit that measures both pH and KH, but if you wish to reach the best CO2 ammounts for plants, you will want to learn these methods. More can be learned here:

  • Surface aggitation allows CO2 to escape into the air. You can hold in more CO2 by reducing surface aggitation, but be causious, since surface aggitation is also the source of O2 for your fish. With too little surface movement, you may cause your fish to suffocate.

  • If you have a place for it, you can attach your air line to a filter (usually canister type) to provide better disolvation of your CO2.

  • You can place your diffusor under your filter intake or flow in many cases to disolve more CO2, but this may sometimes make certain filters rattle.

  • A ceramic diffusor seperates the CO2 into super tiny bubbles that disolve more CO2 into the water before reaching the surface.

  • In areas that have very soft water, I would recomend adding two or three teaspoons of Baking Soda instead of one.

  • A bubble-counter/trap can be made by the following method and can help keep yeast out of your tank in the case of a bottle tip-over:

Copyright 2007
Write Up by T. Fells