Hi Mike.
Don't be alarmed, my drop checker goes from blue to green before my CO2 comes on after water change

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Just bear in mind that you did a 50% water change, so the 50% that was left in the tank still has the residual CO2 left over from the night before, which is what your DC is telling you.
This is nothing to worry about

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oswoldy said:
Also I assume that my plants are using the co2 injected as otherwise the dc would turn yellow if they were not?
I think you answered your own question there Mike

, providing you're not losing too much with any surface agitation.
My tank took about 2 weeks to adapt to it's new CO2 and EI dosing regime, after that the growth rates went nuts

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Chris