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DIY CO₂ Injection: The Yeast Method

By Thomas Narten

INTRODUCTION

Warning: This setup may not be suitable for everyone. It requires regular fiddling and the uncertain CO₂ injection rate may lead to pH fluctuations that can kill fish (especially if you have soft water). My water is fairly hard (GH 140-180 ppm) and the pH varies from 7.8 with no CO₂ to 7.0 with CO₂. Read the FAQ on plants and CO₂ injection before attempting this. CO₂ is an important contributor to robust plant growth, but it is advisable to understand the big picture before going this route. It is particularly important to understand the relationship between pH, CO₂ concentration and carbonate hardness. Note also that CO₂ injection is a complete waste of time if one has inadequate light, as is typically the case with single-tube strip lights sold at most aquarium stores. One bulb usually produces an insufficient quantity of light, and typical bulbs do not produce the "full spectrum" of light plants require.

On the other hand, the described set up is an inexpensive way to observe first hand the benefits of CO₂ injection on plant growth. I personally had never seen what CO₂ could do for a tank before trying it myself, and would probably never have purchased a conventional CO₂ injection system because of the high cost. In fact, I still haven't purchased one. I find the yeast method adequate for my current needs. I have been using it now for 8 months and have managed to acquire and successfully grow over 25 species of plants with only a handful of failures. Here are the steps.

CONSTRUCTING A CO₂ GENERATOR

Note: Other folks have had success attaching the tube directly to the resealable cap. Simply poke or drill a hole through the cap, stick the tubing through it and seal it with the aquarium silicone sealer. The variations are endless.

BUILDING A CO₂ REACTOR

More recently, I constructed a "sophisticated" reactor. The idea is to trap the rising CO₂ bubbles before they reach the surface and escape. Doing so provides two benefits. First, less escaping CO₂ means more CO₂ in your tank. Second, the trapped CO₂ bubbles act as a (very!) crude gauge of how much CO₂ you have in your tank. If a lot of CO₂ is trapped, the amount of dissolved CO₂ in your tank is likely to be high. If little CO₂ is trapped, your rig may not be producing CO₂ at a high enough rate to replace that used by plants or escaping from your tank.

My reactor consists of a small clear plastic bowl that flower pots are placed on ($0.40 at any gardening store). I cut it in half, glued a clear piece of plastic to it and wedged it between the filter tube and aquarium glass just below one of the powerheads. The cutaway portion faces down creating a trap for rising CO₂ bubbles. It is best to use clear plastic so that you can see how much CO₂ is accumulating in the reactor.

Other folks have had success with a more sophisticated reactor that actually circulates water in the reactor in order to dissolve the CO₂ faster. A nice reactor can be made from the wide end of a gravel cleaner. The 1/2 inch tubing attached to such cleaners connects directly to the exit of many powerheads. The reactor catches the CO₂ bubbles, while the water from the powerhead churns and dissolves the CO₂ and then flows into the tank.

Tetra also markets a "CO₂ Bell", a simple reactor with suction cups that attach to the side of the tank. Since it is ready to go, it is the easiest to use, but finding them at local stores may be problematical, since few stores seem to market CO₂ injection related products.

If you use a cannister filter, another possibility is to let the CO₂ bubble into the filter intake. The CO₂ will completely dissolve by the time it exits the filter. The "gravel cleaner" reactor described above can be used to catch the CO₂ with the hose connecting it to the filter intake.

A SOURCE FOR CO₂

Using the above mixture, I regularly get 10-14 days of strong CO₂ production, at a rate of one bubble every 3-7 seconds. When the system stops bubbling (the drop in gas production is often precipitous), dump out everything and mix up a new concoction.

HINTS & DISCUSSION

2) Hot water contracts when cooled. Actually, the hot air trapped in your rig contracts when cooled. If your mixture is warmer than room temperature, it may suck water out of your tank before it starts producing enough CO₂ to reverse the flow. If your CO₂ rig sits below your aquarium, bad things could happen.

One way to prevent problems is to insert a standard air valve between the mixture and the airstone. Right after mixing up a new batch, close the valve. Once the rig is producing CO₂ again, open the valve.

Rather than bother with a valve, I simply keep a spare 2-liter bottle on hand. When the current batch looks like it's starting to peter out (or before, if you don't want to have your tank's pH go up as CO₂ production wanes), mix up a new batch, but don't connect it right away. Let it sit in a corner for 12-24 hours and then hook up the new mixture when you know it's going strong. That way there is no interruption in CO₂ flow through your tank.

Another possibility is to connect two separate mixtures together using gang valves (e.g., use a T-connector to attach both mixtures to the same airstone). Then, mix up a new batch in one rig while the other one is still going. The idea is to always have one of the mixtures going at full steam so that when one mixture quits, the other produces enough CO₂ to keep your plants happy.

3) Hot water kills yeast. If you dissolve yeast in hot water (like what comes out of my gas water heater), you probably won't get any CO₂ from your setup. If you've mixed things up properly, you should get CO₂ production within a few hours, possibly more quickly; I've sometimes had things going 30 minutes after mixing a new batch. Here's what I do:

1. Fill a 2 cup measuring cup with hot water, and dissolve the sugar in it. If you use a cup with a spout, you can pour the mixture directly into the soda bottle without a funnel (one less gadget to clean!)
2. Add three more cups of cold water to the mixture and shake it up.
3. Dissolve the yeast in a cup of cold water and add to mixture. I've only used bakers yeast, but others should work too.
4. Add more cold water to mixture to fill to desired level. Shake the mixture well.
5. Let sit at room temperature for a few hours until the mixture reaches room temperature. That's often all the time that is needed to get initial CO₂ production going. I feel safer connecting my setup to the tank *after* I know that it is producing gas and won't suck water out of the tank.

5) Nicholas Plummer reports that most people who advocate using a cannister filter as a reactor seem to have Eheims. When he tried bubbling gas into the intake of a fluval 203, the gas made the impeller rattle loudly when a bubble formed at the top of the cannister. He suspects that the slightly poorer design of the Fluval is to blame.

Even without funneling CO₂ into the filter, he gets the same effect (but to a lesser degree) late in the day. The water becomes so saturated with oxygen from photosynthesis that it starts to bubble out in the cannister. It reenters solution during the night and is always completely gone by the next morning. Perhaps the slight agitation of the water entering the cannister through an eheim surface extractor causes oxygen to precipitate out of solution.

7) Some folks appear tempted to turn off the CO₂ production at night and back on again in the morning. After all, plants consume oxygen at night and produce more CO₂. This is a mistake. It is better to keep the tank's CO₂ concentration relatively constant because a drop in the dissolved CO₂ concentration leads to an increase in pH. Neither plants nor fish seem to appreciate rapid pH swings.

8) Several persons report the appearance of an apparently harmless white film on their CO₂ airstones. I have not witnessed this in my tank and am not sure of its cause.

9) I am still experimenting with exact ratios of sugar, yeast and water. In my system, the rate of CO₂ production is relatively steady for a number of days, but then drops precipitously (from several bubbles a minute to no production in 24-48 hours). I suspect that the yeast exhausts some trace element or other critical ingredient. Yeast needs a number of elements, including nitrogen (e.g., ammonia but not nitrate), magnesium, phosphorous, potassium, calcium, zinc, iron and copper. My initial hypothesis was that a buildup of alcohol limited yeast production. However, a homebrewing friend reports that it would take roughly 3.75 cups of sugar in a 2-liter bottle to produce enough alcohol to reach a concentration of 15%, the point at which no strain of yeast can survive. As another data point, I have been running a mixture now for over six weeks that is still producing an adequate amount of CO₂. I have no explanation for the longevity of the current mixture. I used the same general recipe that previously worked for only two weeks.

10) It should be possible to culture a new mixture using yeast from a previous batch. Simply save the sludge that collects at the bottom of a mixture. The top layer of sludge, which should be a bit lighter in color, consists of live but dormant yeast cells. The darker layer underneath is dead cells. I have not tried this myself. For more information, look towards the homebrewing field.

RESULTS

• 55g tank.
• 2 40W bulbs (two 18-month old tritons).
• Temperature held at 78 degrees.
• UGF driven by 2 Aquaclear Powerhead 301s.
• Layer of floss between gravel and UGF plate.
• Some plants in gravel, some in "plant plugs", others in clay/plastic pots containing potting soil.
• Hard, alkaline water (tap pH 7.6+, GH 140-190 ppm).
• 2 corys, 3 kribs, 5 otocinclus affinis, 2 pearl gouramis, 2 blue rams, 4 medium angels and an angelicus cat.
• DIY yeast CO₂ rig, airstone, and plastic CO₂ bubble trap (not connected to powerhead)
• Regular addition of trace elements with iron.

I recently counted over 25 species of plants in my 55g tank. About 15 species I've had for eight months, the rest for about four. Most of the plants are doing extremely well. My major successes include hornwort, vallisneria, pennywort, elodea, foxtail, wisteria, ludwigia, java fern, rotala rotundifolia and both hygro. polysperma and carymbosa, which I throw out regularly.

Other successes include willow moss, several swords, various cryptocorynes, alternanthera lilacinia, rotala macrandra, sagittaria, plus a few things I haven't been able to identify. My anubia nana doubled its size in four months, more growth than the entire previous year. It's growing a new leave every week. Last month, it bloomed for the first time.

Some of the plants are not doing much, but they haven't died and rotted either. Part of the problem is that I don't have enough light reaching the bottom. The faster growing plants outcompete everything else. I have also witnessed a number of plants near the bottom struggling weeks to grow a few inches (even when under direct light), but after reaching a height of four or five inches, growth takes off. I'm convinced now that more light would help.

My only true failure is cabomba. I had it growing very well for 2-3 months (I was throwing it away I had so much), but then growth just stopped. The plants got stringy looking and the leaves fell off. Since then I have learned that I probably don't have enough light for cabomba in my tank and my water is warmer (78 degrees) than ideal.

It is now apparently also the case that my tank doesn't contain enough nitrogen to keep all plants growing at full steam. In the three months since purchasing a sensitive Lamotte nitrate test kit, nitrate levels have always registered zero. It is also apparent that some plants are growing at the expense of others. For example, elodea and foxtail growth was prolific at first, but now grows only slowly.

SUMMARY

Hypertext translation by E. Olson from Revision 3, 4/22/94

Note: This document can be freely redistributed to other mailing lists, reprinted in local fish club bulletins, etc. as long as it is reprinted in its entirety and proper credit is given. If you want to reprint it, please contact me first in order to get the latest version. Further comments and suggestions are always appreciated.

Revision 3 of this article appears in the May-June 1994 issue of "The Aquatic Gardener" (TAG). TAG is a publication produced by the Aquatic Gardening Association, which is dedicated to the dissemination of information concerning the growing of aquatic plants. Membership includes the bi-monthly TAG journal. For a one-year membership, send $15 (US dollars) to:

	Up to CO2 Plants The Krib	This page was last updated 29 October 1998