CO2 Discussions

[Zoe]

My biggest aspiration when starting with plants was to "someday" get CO2. I didn't know much about, and it seemed quite confusing and expensive. I made lots of mistakes, and it took me a while to get the balance right, but I'm finally at a point where I'm happy with the CO2 I have.
I'm sure I'm not the only one who is/was confused about CO2, so I'd like to share my experiences with it in the hopes that someone else may benefit. I'm by no means an expert, so by all means, I welcome you to provide your own thoughts and facts and opinions.

Why is CO2 important for a planted tank
Carbon is the building block of life. It's found in all living things, and is a major part of photosynthesis. As such, it is essential to inject CO2 into your tank water if you want optimal plant growth. And although some plants will grow okay without the addition of CO2, all plants will benefit from it.

Different methods of CO2 injection
There are two main ways of getting CO2 into your tank. The first is a DIY reactor, and the second is a pressurized canister.
With DIY, you combine yeast, sugar and water - which produces CO2. Take a 2L rinsed coke bottle, fill it 2/3 of the way with warm water, and add two cups of sugar. Shake until the sugar is disolved. Add 1 tsp of baker's or brewer's yeast. Take the cap, and drill a hole in it that is slightly smaller than the width of some airline tubing. Cut the airline on an angle, to create a point, and pull the point through the hole in the cap with some pliars. And voila, no need to seal the hole with silicone! Put the cap on the bottle, and you have a CO2 reactor.
Pros: Inexpensive, simple, and great for smaller tanks.
Cons: Needs to be refilled every 2-4 weeks, can be messy if the mixture gets into the tank (drunk fish!), unsightly if you can't hide it behind your tank.

With pressurized CO2, you actually need a canister. They come in various sizes, and typically you will find that the 10 lbs size is most common for planted tanks. 10 lbs will last for 12-16 months, depending on tank size, lighting, etc. They also come in 3 and 5 lbs sizes, and those will not last as long. Attached to the canister, you have a regulator valve, which indicates how much CO2 is in the canister, and how fast it is coming out. Then you have a solenoid (not necessary, but allows the canister to be turned off at night to conserve CO2), needle valve (which further controls the amount of CO2 exiting the canister), and possible a bubble counter. A bubble counter is filled with water, and as the CO2 comes out, you can count how many bubbles are being injected per second. The norm is 1-3 bubbles, but this can vary.
Pros: Very effective, high and consistent CO2 yield, and the canister only needs to be refilled with CO2 every year or so.
Cons: Expensive (100-300$ initial set-up, 20-30$ yearly to fill up the tank), the canister is heavy, and needs to be brought to store that can fill up the canister.

The third method, which I've tried but is unpopular, is the Carbo-Plus block. This is basically a flat black plaque which fits between two pieces of metal, and is placed inside the tank. A control box outside the tank controls an electric current, which releases CO2. The block is supposed to last from 2-12 months, depending on who you talk to.
Pros: Simple
Cons: Expensive (unit is 100-150$, each new block is 30-60$), and doesn't work!
Some people swear by this thing, but in my experience, it will work fine for a little while, and then will crap out. Not worth the $. If you do try this method, start it on the very lowest setting. I started it on a low setting, and it still plunged my pH from 9 to about 5.5, and killed one of my angelfish.

Which method is right for my tank?
Anything bigger than 40 gallons will require pressurized CO2. Anything over 3.5 watts per gallon will require pressurized CO2. The exception to the rule is large, low-light tanks, which are fine with 2 to 4 DIY reactors (one refilled each week to ensure consistent CO2).

How do I get the CO2 into the waters?
The reactor and the canister provide CO2 gas. If you put the end of the airline into the tank, bubbles will come out, rise to the surface, and pop. This is NOT what you want, because most of the CO2 is just escaping into the air. What you're aiming for, is to get the bubbles to disolve completely before they can hit the surface. In my experience, there are four methods that work well (well, I'm sure there are more, but these have worked for me). For DIY reactors, a glass diffuser is great. These diffusers have little white disks, which force the CO2 bubbles to break into tiny CO2 bubbles, which can disolve before they hit the surface. These cost 6-10$ on e-bay.
You can also use a ladder, which looks like a plastic maze. The maze, placed inside your tank, fills with water. The bubbles have a long way to travel in the maze, giving them time to disolve.
The following two methods have works great for me with pressurized, but also work fine with DIY. The first is a simple powerhead. You can get these with an attachement that allowed an airline hookup. The CO2 bubbles from the airline are broken up by the water pump's impeller, allowing them to disolve easily before reaching the surface.
My current favourite is a cylinder, about 6" high, with a spiral inside it. You hook the CO2 airline into the cylider. Then you attach a water pump (a weak one - too powerful, and it won't work), which pushes water into the cylinder and creates a vortex, in which the bubbles spin until they disolve.

What is the relation between CO2 and light, and fertilizers?
It's all about balance. More is not necessarily better. For example, I have 130 watts of light over my 30 gallon tank. Great, right? Lots of light? Wrong. I had a huge algae outbreak. The reason being that I only had two DIY reactors. With so much light, the plants were trying to grow like crazy, but were not getting enough carbon to stay healthy. When the plants started to suffer, the algae took over. With pressurized CO2, that might light would have been fine - the plants would have been able to grow quickly and healthily. Fertilizers are another part of the balance - not enough, and the plants suffer. Too much, and the algae explodes.

CO2 vs O2
The general idea seems to be that CO2 is bad, and O2 is good. As mentioned earlier, CO2 is an important part of life, so nothing could be more wrong. Breathing CO2 without O2, if you're a person, is bad But plants basically 'breathe' CO2 during the daytime, and produce O2. [At night, plants take in O2 and release CO2, which is why you turn off your CO2 canister at night (if you have as solenoid).] This is one of these things that you want to balance properly. O2 in the water is usually achieved by water agitation / breaking the surface. Things like HOB filters and bubble wands are used to break the surface, allowing O2 into the water. Too much water surface agitation, and you will LOSE your CO2. Not enough, and your fish won't get enough O2. The idea is for your plants to use the available CO2, and your fish to use the available O2. Unless your tank is overstocked, then a simple HOB filter is enough water agitation. A bubble wand or air stone tends to be too much agitation (depends on the actual bubble size and how powerful the air pump is). Basically, you want a constant stream of water that has contact with the air (like a HOB filter) without breaking up the entire surface of the water.

Testing, Testing
Er, I'm one of those people who doesn't test for anything. I have no idea where my CO2 levels are, and only test for things like pH, kH, GH, when something is amiss in my tank. Someone who tests vigourously can probably add some valuable input here, but my method is to toy with it until it works. Plants are hardy, and I have yet to kill a plant because of an imbalance. The downside is you get lots of algae if you mess up, but a handy CO2 scraper and a siamese algae eater from my 90 gallon tank make short work of it.

Dangers of injecting CO2
The risk of hurting your fish or plants with injected CO2 is minimal. With pressurized CO2, the only danger lies in injecting too much CO2, or starting out too high. If you have a low GH or kH, the idea is that your pH can crash. Your pH can drop, but unless you have extremely low GH and kH, and start out at four bubbles per second, the risk is very small. Just start slow, test your pH, and invest in a solenoid so you can turn your CO2 off at night and further diminish the risk of any pH fluctuations.
With a DIY reactor, the sugar, yeast and water mixture can sometimes get back into the main tank water. Obviously, this isn't something you want to happen, but it won't kill your fish unless a lot of the mixture gets into a relatively small tank. You can remove this risk by being careful not fill the bottle more than 2/3 with water. The risk to the plants, with DIY, is unstable CO2. This won't result in any deaths or anything, but you may get some algae growth if you have a high CO2 yield one week, then a normal CO2 yield for one week, and then a low / very low yield the next week. The solution? Use two or more reactors, refilling one every week.