CO2 is perhaps the most important nutrient in a planted tank. Carbon contents in tank water can vary depending on fish load (respiration), surface agitation (gas exchange) and of course plant uptake. In low light tanks CO2 is not necessary, the speed at which the plants grow is slow enough that most nutrients including CO2 are being introduced into the water faster than the plants are consuming them. Plants have a hard time absorbing CO2 from water, they do it a lot more efficiently from the air which is why many plants will start to grow considerably faster when they reach the surface. Adding CO2 to a low-moderate light tank will speed up growth and allow you to grow many plants that would otherwise do poorly without CO2. With higher lighting, CO2 levels become important as the tank quickly becomes CO2 limited and algae starts to take over.
Methods of adding carbon to a tank
Carbon can be added to a tank in a couple of different ways. The most common are pressurized CO2 cylinders, DIY CO2 and liquid forms of organic carbon. CO2 in gas form is more efficient than liquid carbon so the first two options are the most common. Cost of a pressurized system, or fear of having a gas tank under your aquarium is what pushes many people towards the DIY method. With a little patience and trial and error the DIY method is cheap, safe and very effective.
What you'll need
A soda bottle
Usually a large one but if you have a little 5 or 10 gallon tank you might want to use a smaller bottle to start with.
Just about any yeast will do, I'll be using "Fleischmann's Rapid Rise Dry Yeast" as this is easy to come by in supermarkets, It's sold in small ¼ once (7gram) packets. Any other brand or even fresh yeast is just as good in my experience. The dry yeast is nice because you can store it for a long time.
Silicon tubing or even CO2 resistant tubing will last longer but plain old airline tubing will do just fine, it will just stiffen up a little over time and eventually will need to be replaced.
Just a regular cheap plastic one will do, they are so cheap that it doesn't really matter if you need to replace it once a year or so.
The first and most difficult thing to do is to attach the airline tubing to the bottle cap so that no leaks occur under pressure. This can be done in several ways, the easiest is to drill a smaller hole than needed and squeeze the hose through the hole with some pointy nose pliers. You can use some epoxy resin or silicon around the outside of the cap to hold the tubing in place but eventually it will come off as little sticks to these bottle caps. If you want to get fancy you can use a bulkhead fitting such as these ones from Tower Hobbies. Next you can cut the airline coming out of the bottle cap leaving 5-6 inches of tubing, this is where we will install the check valve which will also allow you to disconnect the bottle from the rest of the airline tubing easily.
For a moderate amount of CO2 I use 2 cups of sugar in 3/4 bottle of water and shake well. Next pour the entire packet of dry yeast in a small glass of warm water, stir well and let it sit for 5-10 minutes. Next pour the yeast in the bottle, shake and let the bottle sit overnight without the cap on it. The reason for leaving the bottle sit overnight is that CO2 production will peak during this time and then level off to a more constant rate. Make sure you do not fill the bottle up all the way, stay at least 3-4 inches from the top.
You can experiment with different mixtures and ratios to get the best CO2 output for your needs. More yeast will increase the amount of CO2 produced, changing the amount of sugar has little effect. You can also try using some baking soda to produce a more stable and constant CO2 output, the idea is that it will raise the PH of the mixture, prolonging the life of the yeast.
Life expectancy of the mixture will depend on many things, mostly the ratio of your mixture and temperature of the bottle. Temperature is difficult to control so that leaves mixture to play with. In general a new batch will last anywhere from a week to two weeks. The mixture will slow down after a week or so and completely die off somewhere after that. This is due to either the sugar running out (having been consumed by the yeast) or the yeast dying off, probably as a result of the high alcohol levels that are produced during fermentation or the resulting PH. Topping off is next to useless so expect to have to prepare a new mix once a week or every other week. Even better if you prepare it the day before and change it regularly before the old one dies. Make sure you clean the bottle very well with hot water between uses, this will also help in prolonging the lifespan on the yeast.
Diffusing CO2 into the tank
Diffusion of CO2 can be done in many ways. The most efficient is to use a powered reactor which can be purchased or built with little more than a powerhead and gravel vac. We'll leave that to another article. Those who have canister filters can feed the airline tube directly into the filter intake and let the filter dissolve the CO2 for you. The easiest method and perhaps the one that allows greater control of CO2 level for a DIY setup is to simply use an air stone. Regular air stones are not too good but you can buy micro bubblers that are made of ceramic, glass or plastic which make a very fine mist of bubbles, perfect for CO2. You can also try the wooden air stones however they tend to have shorter life spans.
Controlling CO2 levels
This is the biggest setback with DIY CO2, well other than having to prepare a new mix each week. You can't simply put a valve on the airline tubing, if you do the pressure will build up to the point that the bottle will explode, leaving a very smelly mess for you to clean up. Depending on the size of your tank and amount of CO2 loss due to surface agitation, you will first have to choose an appropriately sized bottle, you can use any soda bottle or multiple soda bottles for larger tanks. You can experiment with different mixtures to find one that is most ideal for your requirements.
If you are using a powered reactor, you can connect it to a timer and have it turn on and off at set intervals to regulate the amount of CO2 in the water. For people feeding into a canister, increasing or decreasing surface agitation using the filter output is probably the only way to change the amount of CO2 in the water. You will essentially be changing the amount of CO2 that is lost due to gas exchange rather than how much is being dissolved in the water. Lastly, those who will be using an air stone can adjust the depth of the air stone, push it deeper and the bubbles will travel through more water to reach the surface, thus dissolving more CO2 into the water. The amount of CO2 produced varies over the life of the mixture, as the mixture becomes weaker over time you will need to make slight adjustments every couple of days to maintain a stable CO2 concentration.
Measuring CO2 levels
CO2 test kits do exist however many people already have everything they need to measure CO2 levels. CO2 levels effect PH in a way that as more CO2 is added to the water the PH will drop. How much the PH will drop depends on the buffering capability of the water, another words your KH level. If you measure your KH and PH you can use the following table to see how much CO2 is in the water. Most of us have a fixed KH level so in most cases testing PH is the only thing you need to know.
Try to aim for a CO2 level of 10-15ppm for a low light tank and up to 30ppm for a high light tank. Anything over 35-40ppm is not advised, too much CO2 will kill your fish and 30ppm is more than sufficient for plants to use.
PH swings and crashes
As you can see from the table above, if your KH is too low you will experience a large drop in PH, often referred to as a PH crash. Fish don't take too well to any large changes over a short period of time so you don't want this to happen in your tank. First of all you want to make sure your KH is at least 3ppm or above. To raise KH use Baking Soda (1/2tsp of baking soda in 20 gallons of water will raise the KH by approximately 1). You also need to be careful the first time you use a DIY bottle or any time you change your diffusion method or mixture ratio. It's pain the first night but it's worth testing your PH level every 3-4 hours for the first 24 hours, this will allow you to work out the PH swing between day and night and make any necessary adjustments. During the night when the plants are not consuming carbon, CO2 levels will rise, aim for a 20-30ppm CO2 in the morning when the lights first come on, by the end of the day cycle CO2 levels will be lower due to plant uptake. Experimenting with surface agitation and diffusion methods will allow you to find the best way to keep PH swings at a minimum and CO2 levels as stable as possible.
Now that you are providing sufficient CO2 for your plants, you might find that other nutrients become limited. As plant growth increases due to the addition of CO2, so will uptake of every other nutrient. Depending on fish load, the next elements to usually become limited are nitrates and phosphates.
- Never try to restrict the flow of CO2 from a DIY bottle, it can explode!
- Do your best to secure the bottle in such a way that it cannot be pushed over onto it's side, this will cause many problems as the yeast/sugar mix will end up in the tank
- Always keep a close eye on PH for 24 hours after having made any changes to your setup
- Keep your KH at or above 3, DIY CO2 is not very stable and lower buffering capacity can lead to larger PH swings
- Don't shake the bottle to try to get more CO2 out of it
- Don't drink the brew :)