CO2 in the air cannot be the cause because it is not high

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Aquaponics at Mediamatic Fabriek: CO2 in tent 70days before 21 feb.2013 - The CO2 in the aquaponics tent was raised to aid the growth of the vegetables. We did it by growing oyster mushrooms on straw in the aquaponics tent. It is actually the growth of the mycelium that produces CO2. Normal outside air is around 400ppm. You can see that the mycelium stopped growing by the end of january. CO2 is back to normal and we should start a new batch of mushrooms. Willem Velthoven

the CO2 concentrations in the air have been low over the past week. you can check the sensor data. We were above 1000ppm in december but now we're around 400ppm. So this can hardly have caused the water hardness.