Air bubblers, either inside or outside a powerhead do NOT lower pH, they raise it by helping drive off CO2. The only way they can possibly lower it is if you have killer aircon and never open the windows and have so much CO2 in your room air I guess, tho' that is very unlikely.
If you have a measured a flat surface with a spirit level to rest it on, the swing arms hydrometers have been tested to be consistently ok accurate. Refractometers are easier to use repeatedly, but hobbyist refracto's aren't as accurate as people like to believe blindly.