Skip to main content

Carbon Dioxide (CO2) in a Planted Aquarium

Carbon Dioxide (CO2) in a Planted Aquarium

by Richard Brown

Aquatic plants need a source of carbon that they can use to convert light into sugar. If there is enough Carbon Dioxide (CO2) they will use it. When their isn't sufficient CO2 they begin to use calcium carbonate, and be slow to grow at best. Calcium carbonate is a water buffer that prevents wild swings in ph. ph fluctuations are bad for the fish that plants co-habitate with and will cause your aquarium to become unhealthy. CO2 injection dramatically increase the growth rate of plants. The rule of thumb is: brighter green an aquatic plant is the faster it will grow; and to see much of any growth out of submerged bog plants, CO2 injection is a must.

Ambient CO2

Aquarium water will have some CO2 dissolved in it by many basic functions. The bacteria in the filter is the main source of CO2. These microscopic organisms will produce CO2 when they preform their half of the nitrogen cycle. Your fish will produce some along with decaying organic material. The water surface will dissolve a little along with the filter action if the return is poured into the tank.

CO2 Dificiency

Aquarium water with out significant plants will have about 5 parts per million (ppm) CO2. As you begin to add more and more plants the CO2 levels will drop. As the CO2 levels drop plants will begin to consume the carbon in calcium bicarbonate. Calcium bicarbonate is a ph buffer that prevents quick wild swings in ph. If you have, or are plannning to have, a medium to heavily planted aquarium then you should diffently concider some sort of CO2 injection. Not only will it stablize your ph but it will save you a lot of money on plants.

CO2 Way Overdone

CO2 is an acid and will lower the ph of your aquarium. CO2 should never reach more then 20 ppm to insure a safety. If to much CO2 is added carbonic acid is formed H2CO3 (this will happen faster at higher temperatures). Carbonic acid is an extremely strong acid only comparable to sulfuric acid H2SO3. It will suddenly crash your ph, killing everything,;just as if you poured car-battery-acid into your aquarium. Your fish will show signs of stress before this happens as either the oxygen gets pushed out of your tank or more likely the ph is way to low by the simple acidic effect of CO2. CO2 can safely be used and isn't so scary as it may seem. As long as you don't pump excessive amounts of CO2 into your system it is safe. A little extra CO2 does wonders for plant growth. Generally, if your fish aren't breathing quickly and the ph is fine, the CO2 levels are also fine.


Measuring CO2 in Aquariums

Scroll to Continue

Some times it is good to know what your CO2 levels is. you can get an approximation by comparing the kh and the ph from a water sample to a chart.

Plant people add so many other thing to our aquariums that lower ph other than CO2, like wood. Since CO2 is a gas we can take a measurement of ph before and after we shake a water sample for a more accurate reading. You can do this by entering the readings at It is not only simpler, but also more accurate then assessing the ph/kh that is often suggested. CO2 levels greater then 5ppm and lower then 20ppm are ideal.

Pressurized CO2

To get the highest rate of growth from your aquarium pressurized CO2 is your best option. Pressurized CO2 tanks can be bought with a regulator and all necessary components to set it up in your aquarium. CO2 is an acid that will drop your ph, a ph sensor can be used to adjust the flow of CO2, so that you maintain a desired ph that is lower then your ph with out CO2 injection. A 2 pound bottle will last a long time, the exact time depends on the size of your aquarium, surface turbulence, and CO2 consumption by plants. The initial setup cost is expensive, but empty CO2 tanks can be exchanged for full ones, at all wielding shops and most breweries, cheaply.

CO2 Generator

CO2 generators will not release a steady flow of CO2. But, they can easily be made with things you probably have on hand, for less than few bucks. CO2 generators use the brewing process to create co2 that can be ducted into your aquarium.

To make a CO2 generator you will need: an airtight container (2 liter bottle), tubing (1/8 inch aquarium air line tubing), silicon sealant that is not mildew resistant (GE #1, or #2), yeast (1 tsp.), sugar (2 cups), warm water water (90>105 degrees F), and a drill (3/16 drill bit). Drill a small hole in the lid of your air tight container that is slightly smaller then the tubing, be careful not to foul the threads. 1/8 inch air line tubing has a 1/8 interior diameter and 1/16 inch wall. The outside diameter is ¼ inch making a 3/16 drill bit perfect. Insert the tubing through the hole. A sealant should be used to guarantee a strong seal, around the outside of the lid. Add sugar, yeast, and add warm water so that the container is ¾ full. Shake the container so that all the sugar is dissolved. The warm water will activate the yeast, but the yeast will brew at room temperature. The mixture can be adjusted to regulate CO2 flow. It may take up to hour to produce significant amounts of CO2. A foam may be produced.

Bubble Counter

A bubble counter let you know how much CO2 is entering your aquarium. If you are using a CO2 generator it can double as a overflow foam catch. They can be made cheaply. You will need: a small clear air tight container (beef bullion jar size), tubing (1/8 inch air line tubing), non- mildew resistant sealant (GE silicon sealant #1 or #2), water, and a drill (3/16 bit).

Drill two holes that are slightly smaller then the tubing through the lid. Insert a piece of tubing about 1/4 inch through one of the holes, this line will go to the aquarium. Insert a piece of tubing through the second hole so that it reaches the bottom of the container. The second tube will lead to your CO2 source. Seal were the holes enter the lid on the outside. Fill the container half full of water and close the lid. Congratulations on your cheap as dirt bubble counter.

3 bubbles per second is recommended for every 10-15 gallons.

CO2 Dissipater

CO2 does not dissolve very well in water. When CO2 bubbles rise to the surface they are not being dissolved in the water. A CO2 dissipater turns big bubbles into small bubbles, that will dissolve faster in water. A store bought CO2 dissipator works the best and aren't very expensive.

If your a do-it-yourself-er on a budget though, you can make one. You will need: an empty fish food container, filter floss (also known and sold cheaper as quilt batting or undyed polyester wool), tubing (1/8 inch air line), a rock, and drill (3/16 bit).

Drill holes many holes through the bottom half of your old fish food container, including the bottom side. Drill one hole just beneath the threads. stuff the container ½ full with quilt batting. Now insert the tube through the hole that you made just beneath the threads. Stuff more quilt batting in the container to hold the tube in the middle. Screw on lid. The home made dissipater will want to float, so a rock can be used to hold it down.

Whether store bought or home made, place your CO2 dissipator in a concealed location. This will prevent the appearance of anything man made in your aquarium. Things that look man made are a distraction and take away from the beauty of your aquarium.

Related Articles