Hi

I have what I think are some odd things happening in my tank.

I have a 125 litre planted tank with a Sera Seramic PH controller connected to a Sera co2 system

The PH Controller has been bought second hand and its my first so Im not sure whats happening is correct or not. Im trying to stabilise a Ph of 6.5 but odd things happen in my tank. When the lights come on the Ph will drop to about 5.9 and steadily creep up during the day to maybe 6.1. When the lights go off at night the ph then shoots up past the 6.5 limit and the controller then starts injecting co2 overnight keeping it around 6.5. As soon as the lights come on again in the morning the ph drops again to 5.9 ish????

Is this normal? I would have thought that with the plants pearling all day the ph would be rising during daylight hours thus needing co2 injection?

Everything else in the tank seems ok and the plants are growing fine.

Kh is 5, Gh 3 and the ph probe has been calibrated ok with the test fluids.

Anyone have any ideas? PS, I havent tried a different probe yet as this would involve buying a new one :-(

thanks

Stu