In waters that are neither strongly acid nor alkaline, (and also in tissue fluids in your fish) most of the carbon exists as bicarbonate ions rather than as dissolved CO₂. This carbonate reservoir is the underlying reason why water holds so much more CO₂ than oxygen...

Let's ignore that bicarbonate for a moment. What about that H+ ion? Does it look familiar somehow? You know that the pH value of water is a measure of the water's acidity or alkalinity. This could be stated another way: the pH is at the same time a measure of the ratio of hydrogen ions (H+) to hydroxyl ions (OH-). At any moment, a minute fraction of the molecules of water are temporarily dissociated, giving a free hydrogen ion with a positive charge (H+) and a hydroxyl radical with a negative charge (OH-). That H+ ion (it's a proton) doesn't remain free for long. It constantly forms and breaks weak bonds with surrounding water molecules, forming an unstable hydronium ion, H₃O, which has a positive charge. Such charged hydroxide ions and protons play essential roles in all kinds of vital acid-base chemistry, pumping ions across biological membranes or regulating cellular acidity, or providing energy in photosynthesis.

Let me emphasize that only a tiny fraction of the H₂O molecules are broken into these two ions. In pure distilled water, which is "neutral," the hydrogen ions (H+) are just equivalent to the hydroxyl (OH-). In water that is "acidic" there are more hydrogen ions, and in "basic" or "alkaline" water the hydroxyl ions predominate. Acidic water will have a pH that tests below 7.0, and alkaline water will test above pH 7.0.

What is pH? Most briefly, the pH (exponens Hydrogeni, though you'll read other interpretations) measures the "negative exponent of Hydrogen" ions. The pH scale runs from 0 (acid) to 14 (base or alkaline), as you know from doing pH tests. The scale is set up by figuring the weight of the hydrogen ions in a liter of water (at 20^oC). The weight of the hydrogen in a liter of absolutely pure distilled water is 1/10,000,000 gram. Instead of all the zeros you can just say "ten to the minus seventh power" or use the inverse logarithm 7 (i.e. 7 zeros) for the exponens Hydrogeni, that is for the "p" of the "H". By comparison, water at pH 6 has ten times as much Hydrogen, that is 1/1,000,000 gram ( i.e. one over only six zeros). Because the pH scale is logarithmic, a drop from pH 6.0 to pH 5.8 is ten times greater than a drop from 7.0 to 6.8. Ten times more stressful for your fish. As you get to an acidity where the weight of hydrogen ions approaches one gram (a pH of 0.0) the scale becomes increasingly theoretical. In the real world, Pepsi-Cola is about pH 3, what with phosphoric and citric acids plus all that CO₂ diffused into it under pressure; but next morning the "flat" Pepsi is much less acidic--— and much less refreshing!

Fish maintain a blood pH that is just on the alkaline side of neutral, about pH 7.7. They are sensitive to wide, abrupt changes in the pH of the surrounding water. So, don't shift the pH more than two points, that is, 0.2 degrees, at a time. Everyone has a favorite number attached to this warning! but the core message is that--— within reasonable limits--— pH stability is what counts for fishes, not an "ideal" pH value.

Some pH links. If you're willing to work through it, the best basic chemist's answer to the question "What is pH?" is found at "General Chemistry on-Line." By the way, this site is good for definitions of any basic chemistry concept. Sometimes it can clarify even my murky brain.

The best fishkeeper's introduction to pH that can be found on the web is at www.thekrib.com once again.

The best explanation in an aquarium-related book is Diana Walstad's, in The Ecology of the Planted Aquarium, 1999, but behind her book and informing her understanding is the great standard college text, Dr. Robert G. Wetzel's Limnology: lake and river ecosystems, which really does explain the pH of fresh waters.

Adjusting pH values can be tricky and rarely leads to long-term success and stability. Sometimes the motivation for lowering the pH is to breed fish that are said to require a pH below 7.0. Attempting to adjust the pH in water that has a strong carbonate buffer can feel like trying to submerge a beach ball. Phosphate pH buffers are dis-commended in planted aquaria, even by the manufacturers. See the results of a search for the ingredients of Aquarium Pharmaceuticals' "Proper pH7.0" in the material on water conditioners. Recent experiments are showing that environmental cues for fishes' gonad maturation are connected to decreases in water conductivity rather than to low pH values themselves. Reducing the total dissolved solids (TDS) is a better way to go; some methods for water softening have their own page in the "Filtration" folder here.

The balance between KH and CO₂ results in pH. The pH can also be considered as the result of the balance between the CO₂ level and the alkalinity or "KH" ("carbonate hardness"). Just as you'd imagine, this interrelationship can be tabulated. One widely-used chart of KH and pH giving corresponding CO₂ levels can be found at http://www.aquanet.de/privat/Massimo/AquPla.htm and at George Booth's "AquaticConcepts" The pH/"KH"/CO₂ charts have some failings, however. The charts commonly used seem to overestimate CO₂ levels, by as much as 20%. They don't take into account various biogenic acids, aside from carbon dioxide. Or any humic acids, tannins and the like. And all bets are off if part of your buffer is based on phosphates. Details have been discussed at length in extracts from the Aquatic-Plants Digest archived at www.thekrib.com. The most important thing-- more important than blindly reading CO₂ values off the tables-- is to realize that the three parameters cannot be disconnected.

There's a good explanation of "alkalinity"--— the ability of buffered water to resist acidity--— contrasted with "true hardness" at "Home of the Rainbowfish", where Adrian Tappin suggests you use the terms "true hardness" instead of "permanent" or "general" hardness (dGH) and "alkalinity" instead of "carbonate hardness" (degreesKH). He persuaded me.

To return to carbon dioxide, when CO₂ is dissolved in water, the carbon can participate in three further forms: some is immediately dissociated as carbonic acid, and it's then rapidly incorporated into bicarbonates and carbonates, both of calcium and of magnesium. You'll follow more material about them in the notes on dissolved minerals.

Your drinking water straight from the tap is likely to have temporary higher levels of all the atmospheric gases, because it has recently been under pressure, until it was released from the tap, and it was probably quite cold. Conceivably, well water could either be supersaturated with CO_2, thus depressing the pH value, or conversely it could be depleted, thus temporarily raising it; if you have well water, letting it sit for 24 hours is always a wise fishkeeping precaution.

I recently tested New York's soft tapwater, straight from the tap into the measuring vial, and I got pH 7.0 or 7.2. Then I took a second sample, capped the vial and shook it vigorously for a full minute. It re-tested at pH 6.2. Either my tapwater the other morning got depleted of CO₂ on its way here in the watermains, or possibly I super-saturated the water with CO₂ (and other atmospheric gases) by shaking it.

About adding carbon dioxide. The current fad for CO₂ injection in planted tanks began in the mid-1990s. The best CO₂ injection primer is once again at www.thekrib.com where you can also get the basic information on the relations of carbon dioxide and alkalinity ("carbonate hardness"). Some do-it-yourself CO₂ infusion hints are in the "Plants" folder here.

Many folks seem to have been swept up in the CO₂ fad without having a clear goal. CO₂ injection is a way to succeed with plants, even where water is hard and alkaline. The additional carbon dioxide dissolved in water forms carbonic acid that reacts with carbonates to dissolve them: thus the water is softened, pH reduced and CO₂ made available to those plants that can't get it directly from the carbonates. Other good changes occur: in anoxic porewater, lowered pH converts ferric iron Fe(II) to its soluble ferrous form Fe(III) and makes it available to plants. But if you don't have enough natural buffering in your water, CO₂ diffusing can result in disastrous pH swings. Cryptocoryne melts in such unstable conditions. Fishes can be stressed. How to avoid that? Why, add buffering, you'll often be told!

Pearling and "lost" carbon dioxide. Carbon dioxide diffusing can result in plants photosynthesizing so strongly that their vascular system is completely loaded. The water too may become saturated with oxygen: it may contain all the dissolved oxygen it can hold at that temperature. Additional oxygen still being produced by photosynthesis can't dissolve into the water, so now it begins to show as tiny silvery bubbles, especially along the edges of blade-like leaves. Very pretty! People see masses of oxygen bubbles forming all over plants that are photosynthesizing furiously. They enjoy the sight, they have visions of champagne and call it "pearling." But the "pearling" bubbles are becoming a fetish, and anxious types have begun to imagine that without the visually reassuring bubbles, photosynthesis isn't taking place. You won't fall into this trap if you remember that a plant only "pearls" when it is producing, not only more oxygen than it can use, but more than it can even currently store, and that the water cannot absorb it at that temperature.

There's another fear developing in the wake of the CO₂ fad, and that is a fear that surface movement, and the exposure of water to air in a biowheel, are "driving off" carbon dioxide, that CO₂ is being "lost." For instance I see in some good printed recommendations (1997) for keeping a culture of green water aerated, "The only problem with the use of vigorous aeration is that carbon dioxide, which is the limiting factor for the growth of the algae, would be driven off to some degree." --and you are recommended to slow the rate of bubbles to a minimum. I hope you see that aeration and an agitated water surface could never "drive off" CO₂ --—only help it achieve equilibrium with the CO₂ level in the atmosphere. In fact, in sunlight, with photosynthesis going strong, a green water culture or a densely planted tank is more likely to become CO₂-depleted, and aeration would allow atmospheric CO₂ to diffuse into the water, rather than the other way round.

A couple of years ago I read, in a piece by an authoritative columnist in a major fish magazine, "When I started to keep fish in an aquarium, everybody installed a pump with an airstone inside the tank to add oxygen to the water. Today we know that this technique reduces the oxygen." So now the fear about "losing" CO₂ is extended even to oxygen!

Vigorous movement at the surface can only ensure that the balance of dissolved gases is maintained. If plant growth in your aquarium should lower CO₂ levels the least bit, CO₂ will naturally diffuse in from the atmosphere to restore the balance. Carbon dioxide is slow to dissolve in water, so the diffusion process will often be slower than the rate of photosynthesis in a densely-planted, brightly-lit tank. Thus pH levels can fluctuate in a diurnal rhythm. Since gas saturation is a temperature-related thing, you can always raise the oxygen levels for the fish and the carbon dioxide levels for the plants, just by lowering the temperature a few degrees. At any rate, don't worry about "losing" any but artificially added CO₂.

To sum up, all surface water movement, including a bio-wheel, tends to bring atmospheric and dissolved gases, including CO₂, into equilibrium. The dissolved CO₂ level depends on and affects pH and temperature. However, in the special case of aquaria with artificially high infused levels of CO₂, this natural equilibrium is being avoided, for the sake of boosted plant growth. Only the additional CO₂ could ever be "lost" to the atmosphere. So, if you aren't diffusing additional CO₂ into your water, then your water is quickly achieving its natural balance of gases.

CO₂, nutrients and light. In any group of factors that are necessary for the growth of an organism, the factor that is in the shortest supply becomes the "limiting factor." This is clear: all the other factors may be present for additional growth, but the one missing necessary factor puts the cap on what's possible. How many cakes can you bake with 100 dozen eggs and one cup of sugar? If plant growth is strong enough in the presence of nutrients and sufficient light, CO₂ could become the limiting factor.

Various symptoms exhibited by plants in sufficient light, with sufficient carbon dioxide, but suffering from the limiting factor of a mineral "deficiency" are well-known: a good description of the range of symptoms is at www.thekrib.com. If carbon dioxide itself is the limiting factor, nutrients and light can still be present in excess amounts, but plants can't use them. Your decision, now that CO₂ diffusion is an available option, is whether you want more surface water movement in order to bring more CO₂-depleted water in contact with air, so that atmospheric CO₂ will diffuse into the water. Or alternately, whether you want to use a CO₂ diffuser to bubble CO₂ through the water, and consequently whether you want to minimize diffusive interaction between atmosphere and water surface.

The additional carbon dioxide will also scavenge carbonates, so the pH will drop, because CO₂ is involved in the CO₂/carbonates/pH balance as well. Still, beyond a certain maximum level of CO₂ some other factor limiting plant growth will begin to operate. In an aquarium with fish and biofilm and decay processes all contributing ammonia and other nitrogenous waste, nitrogen is unlikely to become that limiting factor. But more intense light may be called for. Or fertilizer. Before you decide to diffuse additional CO₂, be prepared for the unforeseen ways you will be likely to upset the natural balance.

My opinion: Given sufficient light, in moderately soft waters where the carbon is not bound up as carbonate, dissolved carbon dioxide is naturally sufficient for plants and is stable in waters that are neither stagnant nor polluted with nitrogenous wastes. But I'm satisfied with modest growth rates, a smidgen of algae here and there, and easy plants.

Before you make a move to infusinging additional CO₂, read Eloy Labatut and Marcos Avila's sensible article at Age of Aquariums ("The CO₂ Fever") which makes the excellent point that your aquarium first needs to be in balance and your undemanding plants healthy and growing modestly. Then you may want to supercharge the system with additional CO₂ and grow some more demanding plants.

In waters that are neither strongly acid nor alkaline, (and also in tissue fluids in your fish) most of the carbon exists as bicarbonate ions rather than as dissolved CO₂. This carbonate reservoir is the underlying reason why water holds so much more CO₂ than oxygen...

Let's ignore that bicarbonate for a moment. What about that H+ ion? Does it look familiar somehow? You know that the pH value of water is a measure of the water's acidity or alkalinity. This could be stated another way: the pH is at the same time a measure of the ratio of hydrogen ions (H+) to hydroxyl ions (OH-). At any moment, a minute fraction of the molecules of water are temporarily dissociated, giving a free hydrogen ion with a positive charge (H+) and a hydroxyl radical with a negative charge (OH-). That H+ ion (it's a proton) doesn't remain free for long. It constantly forms and breaks weak bonds with surrounding water molecules, forming an unstable hydronium ion, H₃O, which has a positive charge. Such charged hydroxide ions and protons play essential roles in all kinds of vital acid-base chemistry, pumping ions across biological membranes or regulating cellular acidity, or providing energy in photosynthesis.

Let me emphasize that only a tiny fraction of the H₂O molecules are broken into these two ions. In pure distilled water, which is "neutral," the hydrogen ions (H+) are just equivalent to the hydroxyl (OH-). In water that is "acidic" there are more hydrogen ions, and in "basic" or "alkaline" water the hydroxyl ions predominate. Acidic water will have a pH that tests below 7.0, and alkaline water will test above pH 7.0.

What is pH? Most briefly, the pH (exponens Hydrogeni, though you'll read other interpretations) measures the "negative exponent of Hydrogen" ions. The pH scale runs from 0 (acid) to 14 (base or alkaline), as you know from doing pH tests. The scale is set up by figuring the weight of the hydrogen ions in a liter of water (at 20^oC). The weight of the hydrogen in a liter of absolutely pure distilled water is 1/10,000,000 gram. Instead of all the zeros you can just say "ten to the minus seventh power" or use the inverse logarithm 7 (i.e. 7 zeros) for the exponens Hydrogeni, that is for the "p" of the "H". By comparison, water at pH 6 has ten times as much Hydrogen, that is 1/1,000,000 gram ( i.e. one over only six zeros). Because the pH scale is logarithmic, a drop from pH 6.0 to pH 5.8 is ten times greater than a drop from 7.0 to 6.8. Ten times more stressful for your fish. As you get to an acidity where the weight of hydrogen ions approaches one gram (a pH of 0.0) the scale becomes increasingly theoretical. In the real world, Pepsi-Cola is about pH 3, what with phosphoric and citric acids plus all that CO₂ diffused into it under pressure; but next morning the "flat" Pepsi is much less acidic--— and much less refreshing!

Fish maintain a blood pH that is just on the alkaline side of neutral, about pH 7.7. They are sensitive to wide, abrupt changes in the pH of the surrounding water. So, don't shift the pH more than two points, that is, 0.2 degrees, at a time. Everyone has a favorite number attached to this warning! but the core message is that--— within reasonable limits--— pH stability is what counts for fishes, not an "ideal" pH value.

Some pH links. If you're willing to work through it, the best basic chemist's answer to the question "What is pH?" is found at "General Chemistry on-Line." By the way, this site is good for definitions of any basic chemistry concept. Sometimes it can clarify even my murky brain.

The best fishkeeper's introduction to pH that can be found on the web is at www.thekrib.com once again.

The best explanation in an aquarium-related book is Diana Walstad's, in The Ecology of the Planted Aquarium, 1999, but behind her book and informing her understanding is the great standard college text, Dr. Robert G. Wetzel's Limnology: lake and river ecosystems, which really does explain the pH of fresh waters.

Adjusting pH values can be tricky and rarely leads to long-term success and stability. Sometimes the motivation for lowering the pH is to breed fish that are said to require a pH below 7.0. Attempting to adjust the pH in water that has a strong carbonate buffer can feel like trying to submerge a beach ball. Phosphate pH buffers are dis-commended in planted aquaria, even by the manufacturers. See the results of a search for the ingredients of Aquarium Pharmaceuticals' "Proper pH7.0" in the material on water conditioners. Recent experiments are showing that environmental cues for fishes' gonad maturation are connected to decreases in water conductivity rather than to low pH values themselves. Reducing the total dissolved solids (TDS) is a better way to go; some methods for water softening have their own page in the "Filtration" folder here.

The balance between KH and CO₂ results in pH. The pH can also be considered as the result of the balance between the CO₂ level and the alkalinity or "KH" ("carbonate hardness"). Just as you'd imagine, this interrelationship can be tabulated. One widely-used chart of KH and pH giving corresponding CO₂ levels can be found at http://www.aquanet.de/privat/Massimo/AquPla.htm and at George Booth's "AquaticConcepts" The pH/"KH"/CO₂ charts have some failings, however. The charts commonly used seem to overestimate CO₂ levels, by as much as 20%. They don't take into account various biogenic acids, aside from carbon dioxide. Or any humic acids, tannins and the like. And all bets are off if part of your buffer is based on phosphates. Details have been discussed at length in extracts from the Aquatic-Plants Digest archived at www.thekrib.com. The most important thing-- more important than blindly reading CO₂ values off the tables-- is to realize that the three parameters cannot be disconnected.

There's a good explanation of "alkalinity"--— the ability of buffered water to resist acidity--— contrasted with "true hardness" at "Home of the Rainbowfish", where Adrian Tappin suggests you use the terms "true hardness" instead of "permanent" or "general" hardness (dGH) and "alkalinity" instead of "carbonate hardness" (degreesKH). He persuaded me.

To return to carbon dioxide, when CO₂ is dissolved in water, the carbon can participate in three further forms: some is immediately dissociated as carbonic acid, and it's then rapidly incorporated into bicarbonates and carbonates, both of calcium and of magnesium. You'll follow more material about them in the notes on dissolved minerals.

Your drinking water straight from the tap is likely to have temporary higher levels of all the atmospheric gases, because it has recently been under pressure, until it was released from the tap, and it was probably quite cold. Conceivably, well water could either be supersaturated with CO_2, thus depressing the pH value, or conversely it could be depleted, thus temporarily raising it; if you have well water, letting it sit for 24 hours is always a wise fishkeeping precaution.

I recently tested New York's soft tapwater, straight from the tap into the measuring vial, and I got pH 7.0 or 7.2. Then I took a second sample, capped the vial and shook it vigorously for a full minute. It re-tested at pH 6.2. Either my tapwater the other morning got depleted of CO₂ on its way here in the watermains, or possibly I super-saturated the water with CO₂ (and other atmospheric gases) by shaking it.

About adding carbon dioxide. The current fad for CO₂ injection in planted tanks began in the mid-1990s. The best CO₂ injection primer is once again at www.thekrib.com where you can also get the basic information on the relations of carbon dioxide and alkalinity ("carbonate hardness"). Some do-it-yourself CO₂ infusion hints are in the "Plants" folder here.

Many folks seem to have been swept up in the CO₂ fad without having a clear goal. CO₂ injection is a way to succeed with plants, even where water is hard and alkaline. The additional carbon dioxide dissolved in water forms carbonic acid that reacts with carbonates to dissolve them: thus the water is softened, pH reduced and CO₂ made available to those plants that can't get it directly from the carbonates. Other good changes occur: in anoxic porewater, lowered pH converts ferric iron Fe(II) to its soluble ferrous form Fe(III) and makes it available to plants. But if you don't have enough natural buffering in your water, CO₂ diffusing can result in disastrous pH swings. Cryptocoryne melts in such unstable conditions. Fishes can be stressed. How to avoid that? Why, add buffering, you'll often be told!

Pearling and "lost" carbon dioxide. Carbon dioxide diffusing can result in plants photosynthesizing so strongly that their vascular system is completely loaded. The water too may become saturated with oxygen: it may contain all the dissolved oxygen it can hold at that temperature. Additional oxygen still being produced by photosynthesis can't dissolve into the water, so now it begins to show as tiny silvery bubbles, especially along the edges of blade-like leaves. Very pretty! People see masses of oxygen bubbles forming all over plants that are photosynthesizing furiously. They enjoy the sight, they have visions of champagne and call it "pearling." But the "pearling" bubbles are becoming a fetish, and anxious types have begun to imagine that without the visually reassuring bubbles, photosynthesis isn't taking place. You won't fall into this trap if you remember that a plant only "pearls" when it is producing, not only more oxygen than it can use, but more than it can even currently store, and that the water cannot absorb it at that temperature.

There's another fear developing in the wake of the CO₂ fad, and that is a fear that surface movement, and the exposure of water to air in a biowheel, are "driving off" carbon dioxide, that CO₂ is being "lost." For instance I see in some good printed recommendations (1997) for keeping a culture of green water aerated, "The only problem with the use of vigorous aeration is that carbon dioxide, which is the limiting factor for the growth of the algae, would be driven off to some degree." --and you are recommended to slow the rate of bubbles to a minimum. I hope you see that aeration and an agitated water surface could never "drive off" CO₂ --—only help it achieve equilibrium with the CO₂ level in the atmosphere. In fact, in sunlight, with photosynthesis going strong, a green water culture or a densely planted tank is more likely to become CO₂-depleted, and aeration would allow atmospheric CO₂ to diffuse into the water, rather than the other way round.

A couple of years ago I read, in a piece by an authoritative columnist in a major fish magazine, "When I started to keep fish in an aquarium, everybody installed a pump with an airstone inside the tank to add oxygen to the water. Today we know that this technique reduces the oxygen." So now the fear about "losing" CO₂ is extended even to oxygen!

Vigorous movement at the surface can only ensure that the balance of dissolved gases is maintained. If plant growth in your aquarium should lower CO₂ levels the least bit, CO₂ will naturally diffuse in from the atmosphere to restore the balance. Carbon dioxide is slow to dissolve in water, so the diffusion process will often be slower than the rate of photosynthesis in a densely-planted, brightly-lit tank. Thus pH levels can fluctuate in a diurnal rhythm. Since gas saturation is a temperature-related thing, you can always raise the oxygen levels for the fish and the carbon dioxide levels for the plants, just by lowering the temperature a few degrees. At any rate, don't worry about "losing" any but artificially added CO₂.

To sum up, all surface water movement, including a bio-wheel, tends to bring atmospheric and dissolved gases, including CO₂, into equilibrium. The dissolved CO₂ level depends on and affects pH and temperature. However, in the special case of aquaria with artificially high infused levels of CO₂, this natural equilibrium is being avoided, for the sake of boosted plant growth. Only the additional CO₂ could ever be "lost" to the atmosphere. So, if you aren't diffusing additional CO₂ into your water, then your water is quickly achieving its natural balance of gases.

CO₂, nutrients and light. In any group of factors that are necessary for the growth of an organism, the factor that is in the shortest supply becomes the "limiting factor." This is clear: all the other factors may be present for additional growth, but the one missing necessary factor puts the cap on what's possible. How many cakes can you bake with 100 dozen eggs and one cup of sugar? If plant growth is strong enough in the presence of nutrients and sufficient light, CO₂ could become the limiting factor.

Various symptoms exhibited by plants in sufficient light, with sufficient carbon dioxide, but suffering from the limiting factor of a mineral "deficiency" are well-known: a good description of the range of symptoms is at www.thekrib.com. If carbon dioxide itself is the limiting factor, nutrients and light can still be present in excess amounts, but plants can't use them. Your decision, now that CO₂ diffusion is an available option, is whether you want more surface water movement in order to bring more CO₂-depleted water in contact with air, so that atmospheric CO₂ will diffuse into the water. Or alternately, whether you want to use a CO₂ diffuser to bubble CO₂ through the water, and consequently whether you want to minimize diffusive interaction between atmosphere and water surface.

The additional carbon dioxide will also scavenge carbonates, so the pH will drop, because CO₂ is involved in the CO₂/carbonates/pH balance as well. Still, beyond a certain maximum level of CO₂ some other factor limiting plant growth will begin to operate. In an aquarium with fish and biofilm and decay processes all contributing ammonia and other nitrogenous waste, nitrogen is unlikely to become that limiting factor. But more intense light may be called for. Or fertilizer. Before you decide to diffuse additional CO₂, be prepared for the unforeseen ways you will be likely to upset the natural balance.

My opinion: Given sufficient light, in moderately soft waters where the carbon is not bound up as carbonate, dissolved carbon dioxide is naturally sufficient for plants and is stable in waters that are neither stagnant nor polluted with nitrogenous wastes. But I'm satisfied with modest growth rates, a smidgen of algae here and there, and easy plants.

Before you make a move to infusinging additional CO₂, read Eloy Labatut and Marcos Avila's sensible article at Age of Aquariums ("The CO₂ Fever") which makes the excellent point that your aquarium first needs to be in balance and your undemanding plants healthy and growing modestly. Then you may want to supercharge the system with additional CO₂ and grow some more demanding plants.