You can let a small sample of water rest overnight in a wide pan, or shake it vigorously in a bottle with a lot of air, to drive off CO2. Then retest pH.
If it rises a lot, I suppose it's possible that your tapwater really has 30ppm CO2. But a 50% water change will only raise your tank from 0ppm to 15ppm, and then only for a few hours. So you'll still need CO2 injection.
Otherwise, you'll need to do some more investigation into the cause of your odd readings.