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September 23, 2019
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Title Carbon Dioxide Tip Sheet

John Tullock

Last Updated



Carbon dioxide is essential to plants. It does not easily dissolve in water, and therefore can be a limiting factor in aquarium plant growth.

Carbon dioxide is essential to plants. It does not easily dissolve in water, and therefore can be a limiting factor in aquarium plant growth. As a rule, fast-growing plants will require more carbon dioxide, as well as more light, than slower-growing varieties. Among the best selections for tanks without carbon dioxide injection are Anubias, Cryptocoryne, Microsorum, Bolbitis and various mosses. “Stem” plants, generally sold as bunches of cuttings, need more light and more CO2. AquaLife EVOKE can be used in low-tech systems to help plants maximize carbon dioxide absorption. (Take care not to exceed recommended dosage.)

                  Because of the investment, I recommend an injection system for any aquarium over 20 gallons in capacity, although even small tanks can benefit.

For systems with carbon dioxide injection:

  • Refillable CO2 tanks are a better option than disposable/recyclable cartridges.
    • Cheaper in the long run, by far.
  • Make an effort to keep the pH at 7.0, because doing so allows easy calculation of the CO2 concentration.
    • CO2 = 3KH * 10(7-pH) Therefore, when pH = 7, CO2=3KH
  • One-half teaspoon of sodium bicarbonate in 20 gallons of water will increase the carbonate hardness by approximately 1 dKH.
  • Test regularly for both pH and KH to insure they remain in the correct range. At pH=7, the ideal KH is 5 to 6 dKH, or a CO2 concentration of 15-18 ppm.
  • Up to 30 ppm CO2 is OK. More is dangerous to fish.
  • Using a timer-controlled solenoid valve to control CO2 flow dramatically reduces waste. Flow should be maintained only while plants receive illumination.
  • Daylength can be anywhere from 8 to 12 hours, depending upon the plants and the amount of ambient light the aquarium receives. Carbon dioxide injection will not improve growth if plants are receiving insufficient light.
  • Carbon dioxide injection increases a plant’s demand for food. Always use either AquaLife LUSH or AquaLife Phyto-Tone at the recommended dosage.
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