Ambulocetus....Increased gas exchange will in turn help to blow of excess carbon dioxide that may have built up in the water. The excess CO2
will drive the pH down. In your case there are three things that could help.
1. Remove all or a portion of the glass top to allow for more water to air contact.
2. Aim a powerhead towards the water surface to agitate it and facilitate some gas exchange.
3. Thirdly and a little more complicated...try to get some fresh air to the tank. Excess CO2
in your house or room will also worsen this problem. One way to check...remove a small dish of water from the tank..check the pH and then place near an open window for an hour or 2 and then check again. If the pH rises significantly then you may have low O2
in your aquarium room. I doubt that the CO2
issue would keep your pH at 7.0 but addressing it couldn't hurt either. HTH