Why add carbon dioxide to an aquarium?

In the aquarium hobby there are planted tanks. These tanks have live plants, and other invertebrates (such as snails). Just like terrestrial plants, aquatic plants need three main things they need to go through photosynthesis: light, micro- and macro- nutrients, and carbon dioxide. The first two of these are described under the aquatic plant tab of this website. The latter will be described here.

If an aquarium is sparsely populated, but there are many plants, the level of CO2 may decrease, to the point where it inhibits the growth of the plants. In a normal tank the fish are inhaling oxygen and exhaling carbon dioxide. So the only big source of carbon dioxide for the plants is the fish. As stated above, if their aren't enough fish, the carbon dioxide levels will run low and inhibit plant growth. Without a proper amount of carbon dioxide available for the plants, there is no way they can grow successfully.

This leads to the question: How can CO2 be artificially added to a tank, to help plant growth? The answer is a simple, easy DIY way to adding and dissolving carbon dioxide into water, and thereby fertilizing the plants.

The 2 liter generator and small filter bottle.

How to add carbon dioxide to an aquarium

If you have ever baked bread, you already now a DIY method of making carbon dioxide. It evolves adding yeast to suagr and activating it with warm water. You see; yeast is a living microorganism that "revives" when added to warm water. Then when you add sugar, a food source, it begins to "eat" it. a byproduct of this very simple reaction is carbon dioxide, that the yeast "exhales". This is the reason your bread has holes in it.

So, we have method that will develop carbon dioxide, we just need to harness this carbon dioxide and deliver it to the tank. This container that will hold your reaction is called a "generator". Most people use a 2 liter, empty soda bottle. This is a great container to use because it is made to withstand pressure from soda, so it will not burst from the carbon dioxide's pressure.

Putting this reaction together, is not difficult at all. Simply throwing some sugar, some warm water, and some yeast into a soda bottle will start the reaction. There is no exactly correct mixture, everyone's need are unique. In general, the warm water should be added last and should fill the bottle up to the curve. Normal people add 4-6 cups of sugar. More sugar normally means the bottle lasts longer, but too much sugar will intoxicate the yeast. About 1 teaspoon to 2 teaspoons of yeast should be added, depending of how much carbon dioxide output you want. More yeast will give a higher output. This is not an exact sciences and the levels should be experimented with. the sugar and warm water should be directly added to the bottle. Then simply cap it and shake to dissolve the sugar. Adding the yeast is more complicated. To maximize the yeast's production, the yeast should be mixed with some warm water and left to sit for 10 minutes. This is called "activating" the yeast. This process helps the yeast start carbon dioxide production quickly. After 10 minutes, simply add the yeast mixture to the sugar mixture and carbon dioxide production will begin.

Now this carbon dioxide needs to be harnessed and brought to the aquarium. This is done by a tight hole in the cap of the 2 liter bottle and airline tubing all the way up to the aquarium. This hole can be achieved with a 3/16" drill bit. Anything bigger will not be tight enough. Silicone or hot glue should be added around the inside and outside of the hole to keep the seal airtight. Also, a check valve should be used to make sure water does not flow back into the 2 liter bottle. This check valve is barely $3 and will help make sure this DIY unit is safe. Be sure to use it. Another important step, before the carbon dioxide is brought to a filter bottle. This is a simple small bottle, filled half way with cold water. The tubing from the 2 liter bottle goes to the bottom of the small bottl, so that the carbon dioxide has to bubble through water. As the carbon dioxide bubbles through the water, some of the slcoholic content made by the yeast is caught in the water, and cannot harm your tank. Once the carbon dioxide bubbles through the water it exits that small bottle through another piece of aitline tubing theat then goes to the tank. so, the small bottle actually has 2 holes, which both need to be sealed. This setup ishard to image, so please, visit my youtube channel and look at my DIY co2 system. It will definitely help. Now the carbon dioxide has been brought to the tank, and bubbles will start to flow to the top of the tank. everything is working fine. However, we are not done. This carbon dioxide then needs to be dissolved into the aquarium water so that the plants can use it. Many people just use an air stone to make the bubbles very small, and hopefully dissolve into the water. However, to be 95% efficient, you need to use a DIY co2 "Reactor". This involves using an aquarium pump to blast at the co2 bubbles and dissolve then into the water. Lots of videos for countless different styles are available on youtube.

The carbon dioxide output should start within an hour after setting up the generator. This gives the generator time to build pressure. After about a week to a week and a half the carbon dioxide output will begin to drop and the generator will need to be remixed. Be careful when opening the generator, because after a week, alcoholic fermentation will have begun and it could smell strongly. Clean the 2 liter bottle out with hot water and simply make another mixture.

My reactor

My reactor
The airline tubing brings the carbon dioxide to the pump, that blasts it into the tube and dissolves it. Look on youtube for its complete description.
Carbon dioxide bubbling into the filter bottle.

Check valve

Check valve
This check valve ensures my aquarium won't drain out if there is a problem with the generator.

Carbon Dioxide Systems

For planted tanks over 75 gallons large, many people use more sophisticated carbon dioxide sytems. These systems are mmore sophisticated than DIy setups. They include pressurized carbon dioxide tanks that use valves to deliver exactly the right bubbles per second unit. The bubbles are then dissolved into the tank using any reactor style. These sytems are more expensive, they can cost 200-300$ and are really just for huge, heavily planted tanks. However, these systems do not need to be re-made every week so many people prefer them.

Things You Should Know

  • When carbon dioxide is added to water it forms carbonic acid, which then releases H+ ions into water. This lowers the water's pH. Drastic pH swings can become a big problem at night. During the day, the carbon dioxide is being used by the plants in photosynthesis. But, during the night, the plants don't undergo photosynthesis and therefore the carbon dioxide builds. This means there are going to be slight pH swings in a tank the adds carbon dioxide. These pH swings are natural, and happen in normal ecosystems. The pH should none-the-less be monitored when DIY co2 is first added to a tank. However, the airline tubing should never be removed from the aquarium, in hopes to avoid this slight pH change. Also, the generator should be re-mixed before the old generator completely stops production. Otherwise, there could be harmful pH swings when a new bottle is applied.
  • The only real way to control the carbon dioxide output is the mixture itself. Do not try to control the amount of carbon dioxide with valves. If you inhibit the amount of pressure allowed to exit the bottle, you could in fact burst the 2 liter bottle and cause water to siphon out of the tank. Simply do not try to control the carbon dioxide pressure.
  • Always use a check valve, unless you want to come home to a flood.
  • Carbon dioxide is not bad for your fish. It will not suffocate your fish. However, low oxygen levels can be very bad. Simply ensure that there is some water movement and your fish will not have problems with DIy carbon dioxide. Surface movement will increase gas exchange and make sure the water stays oxygenated. This is only a real problem during the night when both plants and fish are using oxygen and the levels may run low.

DIY CO2 Setup

DIY CO2 Setup