Point a powerhead at the surface of the water. Having more oxygen into the water column helps with ph. This being said, ph levels will normally be lower in the winter months due to our homes being shut up and heaters running. This level also fluctuates during the day.
My normal suggestion to people who have 'ph issues' is to not do anything besides bring more oxygen into the water column. There are ph buffers out there, but when chasing this magical perfect number of ph we more than often cause more issues! Your system will find a spot where everything is happy in terms of ph. As long as nothing is being dosed into the tank, everything will be just fine as long as there is oxygen going in, co2
going out, and weekly water changes to replace essential elements and lower nitrates.