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The aquarium plants and CO2

Carbon is the fundamental element that all life on this planet is based. Plants are no exception. Since plants have no way of getting to their food sources, nutrients have to beobtained from their surrounding environment. Plants use many macro and micronutrients, carbondioxide(CO2)being one of the primary macro nutrients. In an aquarium the limiting factor sare most likely tobe (in order): light, CO2, micro nutrients (trace elements), and macro nutrients.Micro and macro nutrients are usually supplied in adequate quantities by fish waste and the addition of fertilizers. Plant suse a process known as photosynthesis to produce the carbohydrates they need for life.

Photosynthesis requires light for energy and CO2 to drive the chemical reactions. The process of photosynthesis requires a specific light energy threshold. In other words,there is a point where light has reached a specific intensity to start photosynthesis.If the light is not bright enough, photosynthesis will not occur. Beyond that threshold and up to some high light level, photosynthesis will run faster and faster. According to known practice, when light levels exceed two wattsper gallon, supplementary CO2 is required for most aquariums. In our planted aquariums, CO2 is present without it being added my mechanical means.

Fish respire CO2 from their gills. Also in an aerated tank, CO2 from the atmosphere is dissolved in the water.This effect is known as atmospheric equilibrium. In nature though, CO2 level sare usually higherthan can be explained by animal respiration or atmospheric equilibrium, and aquatic plants have evolved to this higher concentration of dissolved CO2 in water. Carbondioxide rich ground water often feeds the streams and natural CO2 concentrations up to several hundred times atmospheric equilibrium are common. In general, aquatic plants like to seeapproximately a concentration of 10-15ppmof dissolved CO2 in their environment. CO2 levels from atmospheric equilibriumare generally around 2-3ppm. (ppm stands for part per million). As you can see,CO2 injection is essential for vigorous plant growth, and even more so with higher light levels. As far afish are concerned, high concentrations, CO2 can block the respiration of CO2 from the fish gills and cause oxygen starvation. Since the gills depend on a CO2 on centration differential between the levels in the blood and the water to transfer gases, high levels in the water will reducethe amount of CO2 that can be transferred. Although different references have wildly varyingvalues for toxic levels, a concentration of below 30ppm is definitely safe.

It is acommon misconception that water can hold only so much dissolved gas and adding CO2 will displace oxygen. This is not true. As a matter of fact, if enough CO2 and lightis present to enablevigorous photosynthesis, oxygen levels can reach 120% of saturation. Even atnight, when theplants stop using CO2 and start using oxygen, the oxygen levels will stay aboutthe same as atypical non-planted aquarium. So reports of people having fish at the surfacegasping for airis not necessarily a result of high CO2 levels, but instead a lack of oxygen inthe water is probablythe culprit.There lationship between light and CO2 levels is important. The diagram at theright explains it conceptually.At low light and low CO2 there is not uch energy to play around with for up or down-regulationof the pools of Chlorophyll or enzymes contained in the plant. If we then add a little more CO2 to the system the plant can afford to invest less energy and resourcesin CO2 up takeand that leaves more energy for optimizing the light utilization - Chlorophyllcan be produced without fatal consequences for the energy. Hence, although we have not raisedthe light,the plant can now utilize the available light more efficiently. Exactly thesame explanation can beused to explain why increased light can stimulate growth even at very low CO2 concentrations.With more light available, less investment in the light utilization system is necessary and the free energy can be invested into a more efficient CO2 uptake system so that th eCO2, which is present in the water, can be more efficiently extracted.

Providing macro and micronutrients to plants is easily done with commercially available fertilizers.It is often a more difficult and expensive task to provide adequate light overthe plant aquarium. Both numerous fluorescent light and halide lamps willproduce sufficient light if supplied with effective reflectors, but in deepaquaria (more than 20 inches) is very difficult to offer enough light to smalllight demanding foreground plants. Based on known experiments, I suggest commencing CO2 addition before any other action is taken! I believe that evenat very modest light intensities you will experience a conspicuous change inplant performance in your aquarium. The exact amount CO2 may always bediscussed but concentrations from 10-15ppm will only improve plant growth. You will probably see that plants, which were barely able to survive before now thrivein the presenceof CO2.