AQUATIC HORTICULTURE		BY KAREN RANDALL

Providing Supplemental CO₂

KAREN RANDALL

As is clearly shown in this photo of Rotala wallachii covered with oxygen bubbles, oxygen supply is not a problem in a properly managed planted tank, even with supplemental CO2.

There are several commercial CO₂ systems available for those who would like to go that route. These systems vary from fully automatic to completely manual. The best of these systems are extremely reliable and fairly easy to install and operate. Be aware, however, that there are other commercial systems that are very high priced for the value you receive. If you are going to buy a commercially available system, do your homework and shop carefully!

For those of you who want to provide supplemental CO₂, but would rather put together your own system, that too is possible, and not that difficult. It costs a lot to set up a tank of even moderate size. When you are talking about a tank of 29 gallons or more, the cost is usually up in the range of several hundred dollars when all is said and done. A homemade pressurized tank CO₂ system will cost under $200 and will be much more useful than many expensive commercially available systems. If you are investing a fair amount of money in your setup anyway, it may make sense to add a pressurized tank CO₂ system at the same time.

Do-It-Yourself Pressurized Tank System

Take the time to do some comparison shopping (see the sidebar entitled “Materials List for Do-It-Yourself Pressurized Tank System”). I have found that the cost of CO₂ supplies varies considerably from one supplier to another. CO₂ canisters also vary considerably in price. With many gas suppliers, you “buy” a tank, but when you bring it in for a refill, you don’t get the same tank back — they trade it for a full tank. This way, they can make sure that tanks are tested periodically, as required by law, for the safety’s sake. You can find even less expensive tanks at fire extinguisher companies that sell used hydro-tested tanks, but make sure you also have a place that will refill these oddball tanks for you. Remember, you will now be responsible for making sure that periodic safety checks are performed on the tank.

My first regulator was purchased from a welding supply shop for about $120. This is an industrial-grade regulator, and is really not optimal for our purposes because the lowest pressure on the output gauge is higher than I would like. For my second setup, I found a store that specialized in winemaking and beer brewing supplies. There, the regulator was less than half the price of my first regulator, and the lower flow was more suitable for my purposes. This was definitely a case where cheaper was better!

Needle valves can be purchased from scientific supply houses and commercial plumbing suppliers (such as Grainger at www.grainger.com). If you don’t have access to one of these companies, check with a doctor, veterinarian, plumber or science teacher for some help locating one.

Materials List for Do-It-Yourself Pressurized Tank System CO2 cylinder (5 pound or 20 pound) — $25 to $100 Regulator — $55 Needle valve — $10 to $40 Air line tubing — $2 Gravel vacuum or small clear plastic bottle — $1 or $7 Total: — $105 to $214

Connect the regulator to the CO₂ cylinder and the needle valve to the regulator. Some people make do with a regular air line gang valve. Don’t do it! You take the chance of dumping the entire tank of CO₂ into your aquarium in the case of an accident. If you would like to run several aquaria off the same CO₂ tank, several needle valves can be cobbled together with plumbing fittings to make a safe, effective manifold. Attach a piece of air line tubing to the outflow of the needle valve. The air line tubing will be fed into your reaction chamber.

While I run my CO₂ systems continuously to maintain as steady a pH as possible, some people feel the need to turn the CO₂ off at night, when the lights are out. If you do this, you must also use a CO₂-safe (not plastic) check valve. Small amounts of CO₂ actually diffuse right out through the tubing. Without positive pressure, the water level in the tubing will back up until it starts siphoning the water out of the tank. Besides the obvious dangers of a wet floor and dry fish, this can also damage your regulator, or even the CO₂ tank itself.

I use Teflon plumber’s tape on all threaded connections to help seal the joints. I have heard some people say that small bits of Teflon can get into and damage a regulator, but if there is positive pressure in the system, and all old tape is removed and replaced each time the tank is changed, I don’t see how it could get into the regulator. Check all connections for leaks by painting on a solution of dish soap and water. If bubbles form, you are losing CO₂!

There are a couple of options as to where to infuse the CO₂ into the system. The first is to insert the air line tubing directly into the intake of your canister filter. In this case, the CO₂ will dissolve in the water inside the filter. There are two disadvantages to this method. First, the bubbles hitting the filter can make a fair amount of noise. Second, you cannot visually monitor the rate of flow unless you add a bubble counter to the system. Some people solve this problem by replacing the filter intake tube with clear tubing, but the filter must still be turned off temporarily to check bubble count.

The method I prefer uses the wide end of a gravel vacuum or a clear plastic water bottle (with the bottom cut off) as a reaction chamber. This can then be attached to the outflow end of your canister filter. The air line tubing from the CO₂ system is then fed up into the bottom of the gravel vacuum. You will be able to see the exact flow rate of CO₂ by counting the bubbles through the plastic. The bubbles will tumble around in the filter outflow inside the cylinder, and be dissolved. I find it helpful to attach a “J”-shape piece of rigid air line tubing to the end of the flexible tubing to help keep it in place within the reactor.

There are a couple of parameters to keep in mind when setting the CO₂ flow rate. First, you want the pH to be between 7.0 and 6.5 or so. Second, the CO₂ levels in the tank should be between 10 and 40 milligrams per liter (mg/L). I suggest staying in the middle of this range for safety’s sake. You will have to make sure that your carbonate hardness is such that you can stay within these guidelines. The recommended levels for carbonate hardness are between 3 and 6 dKH.

There are no hard and fast rules to follow as far as how much CO₂ should be added to the tank. It will depend on the size of the tank, how much CO₂ is being driven off by surface turbulence, how heavily the tank is planted, the stocking levels, and lighting. Start slowly and work up to the optimal level of CO₂. I would suggest starting with a flow rate of one bubble every five seconds and work upward from there.

During the period when you are adjusting the level of CO₂ infusion, monitor the pH very carefully. You should check the pH every couple of hours for the first day or two. Then, when you think you have it adjusted properly, continue to monitor the pH three times a day for another week. If you go a week without needing to adjust the flow rate, you’ve probably found your optimum level. Even then, it is important to check the pH regularly to avoid problems.

Here lies the difference between this system and an expensive, fully automated system. In a fully automated system, you set the desired pH level and the system automatically uses just the right amount of CO₂ to maintain your chosen pH.

Until you are thoroughly familiar with the use of CO₂ and are confident of the infusion levels, I would run an airstone on a timer set to come on when the lights go out. This will drive off excess CO₂ and avoid major pH drops overnight. As time goes on, you will find that if the carbonate hardness (KH) is adequate (between 3 and 6 dKH) and the plants are growing well, and if stocking levels are moderate, the pH will remain more stable without the use of an airstone overnight. But, until you are sure of yourself and your system, it is better to be safe than sorry.

Pop Bottle Yeast Reactor CO₂ System

If you’re just not convinced yet that you are willing to spend the money on a pressurized tank system, or if you have only a couple of small aquaria that need supplemental CO₂, here’s a system that anyone can afford. The amount of CO₂ produced is small, so it’s hard to hurt anything if you make a mistake.

Start with a plastic bottle approximately 2 liters in size. I have started using juice bottles rather than soda bottles because they are a little sturdier and don’t collapse when you try to pick them up with one hand.

You need to make a hole in the cap of the bottle big enough for a piece of air line tubing. I’m sure this could be done with an appropriately sized drill bit. I use a large nail that I hold in a pair of pliers in the flame of my gas stove. When it is hot, I simply melt a hole in the plastic cap. It takes a little pressure, and I may have to reheat the nail a couple of times, but I get a nice smooth edge and a tight fit.

Make the hole just slightly smaller than the diameter of the tubing. Cut the end of the tubing on an angle and use your pliers to pull the tubing through the hole in the cap. The fit should be quite tight. This will give you a fairly CO₂-proof seal all by itself, but you don’t want any CO₂ to leak out around the edges, so it is best to seal the tubing in place. I have found that aquarium sealant doesn’t do a very good job. I am able to get a good solid seal using a hot glue gun.

At this point you need to decide where you are going to feed the CO₂ into your tank. One method is to set up a diffusion bell made of an inverted baby food jar glued to small suction cups. The suction cups keep the jar suspended on the back of the tank. Feed the end of the air line tubing into the bottom of the jar. CO₂ collects in the jar and dissolves in the tank water. Commercially available diffusion bells are inexpensive and work fine too. Another possibility is to feed the end of the air line tube directly into the intake or outflow of a canister filter. While I don’t really like this option for pressurized tank systems, with the smaller amounts of CO₂ coming from a yeast reactor, this method is easy and effective.

The mixture I use in my yeast reactors is 2 cups of sugar and 1 teaspoon of yeast in an almost full bottle of lukewarm (not hot) water. This mix lasts up to four weeks before half the mixture needs to be replaced. Your results may vary depending on the water supply, the yeast used, temperature and other variables. If your water is very soft and the pH is low, you may find that the yeast produces too much CO₂ in the beginning, then exhausts itself quickly. In this case, the addition of a teaspoon of baking soda will slow down and even out the reaction. Play around until you find what works best for you. People who use yeast reactors on an ongoing basis usually start a new bottle a day or two before they expect the old one to slow down to maintain as consistent a flow of CO₂ as possible.

Make sure to either keep your yeast reactor higher than your tank, or use a CO₂-safe check valve between the reactor and the tank. Otherwise, you can end up siphoning your aquarium water out into the bottle with unpleasant consequences!

This system is simple enough for anyone to put together, and extremely inexpensive. If you’re intrigued with the idea of supplemental CO₂, but are reluctant to lay out the money for a bigger system, this is a good way to get your feet wet. A yeast reactor takes a little more maintenance than a semi-automatic system, but it is perfectly capable of producing the amount of supplemental CO₂ needed by your plants, particularly on a smaller tank.