PH swing..

What are you measuring with, a PH probe? How long ago was it calibrated? Do you know where your Mg, Ca, & Alk levels are at?

 

When you say its been calibrated twice...do you mean in the last 10 months or over the last 5-6weeks since you noticed the increase? PH probes require regular calibration to be accurate and over time (1-2 yrs) they can become defective. Hard to say what might cause it to raise unless it's something that you're dosing which might cause a decrease in CO2. Some supplements will raise it...especially if you are dosing kalkwasser. An increase in O2, or decrease in CO2 could raise it as well....but usually in the winter it lowers since your house is sealed up without having the windows open.

I would double/triple check your readings before you take any action.. You could dose some vinegar to bring it down if needed but do so slowly.

Heres some info from a very respectable chemist:

http://www.advancedaquarist.com/2002/6/chemistry#section-9