Mastering Freshwater Aquarium Ecosystems
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Aquarium Equipment (Cont.)

You must take care when adding CO2 to the water, because it will combine with carbonate to form carbonic acid which will lower the pH of the water. It is best to only run CO2 during the day. During the night plants produce CO2 and use up oxygen, so you should not inject CO2 at night.

For controlling CO2 output you can use an electronic solenoid control connected to a timer that can also control the turning on and off of the lights. CO2 is often added to the aquarium by injecting one bubble per second (sometimes more or less depending on the size of the aquarium) into a CO2 reactor that provides a method of exposing CO2 to the aquarium water for an extended period. Another way, is to inject CO2 into a powerhead or sump pump intake. Some hobbyist use a fine bubble defuser to add CO2.

Not all plants respond the same to CO2, some will not show any noticeable growth difference, but many will need culling or trimming once a week. It is possible to inject too much CO2 into the aquarium, which can lead to fish mortality. You must monitor the fish for signs of stress when using CO2, and discontinue use if it detected.


Most aquarium thermometers are accurate to within 2ºF (1ºC). Thermometers are available in many styles. The common styles include:

  • Small glass floating, sinking, suction cup, hang on the rim
  • Tape style, attached to the outside glass
  • Digital, probe is placed in the water

Some heaters like Hagen’s Fluval E series of heaters have a built in thermometer. You set the temperature you want it to maintain and the heater will display the temperature of the water. If the temperature drops three degrees below what you set it at, the display turns blue and indicates the current temperature, and if it is three degrees over, the display turns red and indicates the current temperature.

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