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[APD] CO2 rates, CF & pH swings

I recently upgraded the lighting in my 50g tank from around 130w to 6 x 
55w CF's (= 330w).

I have a CO2 system with pH controller but, with my previously lower 
lighting levels, I ultimately disconnected the solenoid valve & just ran 
the CO2 "open-loop" at a nominal rate.

This was fine to keep the plants "ticking over".

Now though, with the new lights, I find that, during the night (lights 
off) the pH falls to around 6.9.  But within 3 - 4 hours of the lights 
coming on the pH rises to around 7.5 (the current pH of a sample of tank 
water separately aerated overnight is 7.6).

Is that because with the increased light, the plants are using the CO2 
faster than it's being provided when the lights are on?

I've scanned the archives & opinion (in the few posts that cover this 
topic) seems divided?

I'm interested to know because I'm considering putting my pH controller & 
solenoid valve back into operation & increasing the baseline flow rate so 
that, when the pH starts to rise, the CO2 injection rate can be much 
higher than I would dare to run continuously.

Regards, Kevin
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