Yes put a light in your sump to come on at night will help stabilize your ph. Carbon dioxide from humans builds up in your house and your tank will absorb it, lowering your ph. Fresh air is your tanks friend.
"As the concentration of carbon dioxide increases in the atmosphere, the increased uptake of carbon dioxide into the oceans is causing a measurable decrease in the pH of the oceans which is referred to as ocean acidification. Although the natural absorption of CO2
by the world's oceans helps mitigate the climatic effects of anthropogenic emissions of CO2
, it also results in a decrease in the pH of the oceans."
Same as your house except on a small scale, in the winter if we get a warm day I always try and get fresh air into my house. I also recommend a ph monitor, not that expensive and it gives you 24/7 ph monitoring, if you are testing often do to ph problems, it will pay for itself in a short time.