Elaine T wrote:
wrote:

What are the signs of lack of co2?

Algae and slow-growing plants.

Whoops. Forgot the big one! A fall in KH but a rise in pH. The rise
in pH is how you know it's not "old tank syndrome." Some plants can use
the carbon present as carbonate buffer in the tank when they're starved
for CO2. The carbonate gets used by the plants in an unusual way that
leaves excess OH- in the tank water so pH rises rather than falls.

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Elaine T __
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