Of all the strange things that I have done over the years for the aquarium hobby the strangest is probably setting up a "home brew" CO₂ reactor. Think about it. I am employing yeast to ferment carbohydrates, not, as any sensible human being would do, to create a mood-altering beverage, but instead, to feed carbon dioxide to a glass box filled with water. This is not the sort of thing one can easily explain to casual visitors to your home.

But this is not the first time I’ve done this. I briefly had a yeast-filled pop bottle feeding a 30-liter gourami tank when I was living overseas. It worked…I think. At least the plants grew so quickly they filled the tank in no time flat. But the water coming out of a Hobart tap is some of the purest in the world: naturally soft with a pH of 6.0 or so. So it was hard to know what effect the very small amount of CO₂ I injected had had, and what was the effect of using soft acidic water in the first place.

When setting up the gourami tank, my then-future wife, who I had recently met in Hobart, accompanied me on a trip one day to an excellent aquarium store south of the town. They had a lovely show-quality pair of Peruvian severums (Heros appendiculatus). So now severums are her favourite fish, and if you saw this pair you would understand why.

So…when we moved to Calgary she wanted me to get her some severums in payment for putting up with the large aquarium in our living room.

But Calgary’s water, with a hardness of about 150 PPM CaC0₃, and a pH of 8.0 or so, is not that good when it comes to growing either severums or the vast majority of tropical aquatic plants. They come from water like that magic elixir that flowed from my faucet in Hobart, not that stuff that the limestone-laden Rockies keeps sending down the Bow River.

So I thought I would to try CO₂ injection once again, this time to lower the pH for the severums and to provide the plants with a carbon source

I bought a couple of plastic wine carboys at a garage sale. Plastic carboys have proven unsatisfactory for wine making, and so most home wine makers now use glass. You can therefore pick the plastic ones up cheap at garage sales. And one-hole rubber stoppers that fit these things are available everywhere wine making supplies are sold.

Into the carboy went a few seconds of pouring worth of sugar, about 20 liters of warm water, and a healthy dollop of yeast. A length of rigid tubing was stuck through the rubber stopper, and a length of air hose was attached to that. The rubber stopper was put into the carboy’s neck, and the free end of the air hose was stuck up the inlet siphon of the Fluval 303 that filtered the tank. The idea, taken from the Internet, would be that the canister filter would serve as a "CO₂ reactor", with its impeller breaking up the gas bubbles so they would dissolve better.

The tank was already heavily planted with Vallisneria spiralis, Cryptocoryne affinis, Ludwigia repens, Echinodorus bleheri, and Pistia stratiotes. But the tank had no other filtration besides the Fluval 303. And I had placed the outlet hose of the filter below the water line so as to avoid losing the CO₂ by surface agitation, as I read you should.

Big mistake!

This first experiment almost ended in complete disaster, as when I got home from work the next day, all five of the young severums looked, for all the world, dead. Their gill covers were stiff and distended, and they were lying on their sides. But there was still some sign of breathing. I yanked the CO₂ line from the Fluval’s intake siphon, got an air pump, and had two air-stones running in the tank immediately. Then I did a pH test…the indicator solution’s color was solid yellow, so the pH (whatever it was) had dropped below 5.5 from its original value of 8.2.

This was obviously too effective!

But miraculously, all the fish survived! By the next morning they all seemed happy as clams. The pH by that time had read about neutral.

After the initial near disaster I have worked out a system that seems to work well, keeping the pH neutral or slightly acidic. I took an old Penquin Biowheel out of the closet. It had been relegated to the junk closet because the water pump it came with could not pump enough water (even without any filter media in it) to keep its biowheel turning, rendering the filter useless. This was my first purchase of a Penquin product, and it is so poorly designed (i.e. non-functional) that I have serious doubts about the quality of all other Penquin products as well; so much so that I won’t bother to buy another. But the water pump of the Fluval 303 is considerably stronger than the Penn Plax’s, and so I thought it could turn the wheel. It worked! I stuck the outlet hose of the Fluval into the hole for the siphon tube for the Penquin. The water then flowed back into the tank under the turning biowheel. I have just enough CO₂ loss through this system to keep the pH in the safe range, provided that I don’t add too much yeast. Every two weeks I drain ¾ of the water from the carboy and replace it with fresh warm water and a good handful of sugar. This keeps the CO₂ bubbling at just the right rate.

The plant growth is very good in all species except the sword plants. I attribute this, with no evidence whatsoever, to a lack of nutrients in the sand (Sil 7) substrate. The books say that sword plants need a nutrient and iron rich substrate.

Remember the "big tank" in the living room whose (admittedly obtrusive) existence led to getting the severums in the first place? Well, this tank held at the time a mixed collection of Lake Malawi cichlids along with some Vallisneria spiralis, Cryptocoryne affinis, and Pistia stratiotes. The tank’s plant filter also had a solid growth of temple plants (Nomaphila stricta). All the plants were doing OK, but the only ones that were actually thriving (in the same way that the plants in the severum tank were thriving) were the temple plants and the water lettuce. But both of these plants grow up out of the water, and so can get their CO₂ from the air.

This led me to think that the growth of the fully submerged plants in the Lake Malawi tank was CO₂ limited. But CO₂ injection is out of the question in a Lake Malawi cichlid tank. These fish are very sensitive to pH changes and need it at a constant 8.2.

However I had grown a little tired of my Lake Malawi cichlids. They are beautiful, yes, but they are also remarkably stupid for cichlids. Where are the great personalities that cichlids are famous for? Not in Rift Lake cichlids in any event. And breeding them was hardly a challenge anymore.

I was ready for new challenges. To try and finally grow healthy, dark-leafed, and flowering sword plants seemed to be what I needed.

So the September auction saw the sale of all my Lake Malawi cichlids. The tank’s gravel substrate was replaced with Sil 9 gravel to which laterite (purchased at Pisces) was added to the lower third. The Lake Malawi rockwork was replaced with some driftwood. And the carboy was set up to inject CO₂ into the tank overflow. Yes, it would be great to have a high-pressure CO₂ system with an electronic dosimeter, but financial realities are what they are.

The tank’s Vallisneria spiralis and Cryptocoryne affinis were replanted in the new substrate along with some new sword plants from Twyla and the Java fern, Anubias, and Ludwigia from the smaller tank.

Then the severums were moved in (they were getting too big for their 125-liter tank anyway) along with a small army of Otocinclus affinis from Pisces and a large school of Corydoras gossei and Corydoras baineno from Birgit.