Make the heater be at 28 C. pH is not a water parameter and it does no matter when testing water, pH is a measure of H+ and OH- ions in water. Test your tank water for ammonia, nitrite and nitrate and then give us the results. If ammonia (NH3 or NH4) and Nitrite (NO2) are both above 0ppm do a 75% or larger water change, and if Nitrate (NO3) is above 20ppm do the same what you would do with ammonia and nitrite.