No they don't increase the pH in the water, they just increase the oxygen content.
If your pH is a little high then I'd first test what your tap water is, and if the tank water pH is higher then I would start looking at the rocks and gravel (if you use gravel).
To check if there is any limestone (a stone that will increase the pH) in your rocks, you need to put a few drops of vinegar on each rock (removed from the tank obviously

) and if the vinegar fizzes that's a sign that either the rock is limestone or has limestone in it. You may want to remove the rocks to see if the pH drops.
You also need to do the same test with a handful of gravel, just put the gravel on a small plate and then add some drops of vinegar onto it, if any bits of gravel fizz that's a sign its limestone.
I had a problem with elevated pH in my SA cichlid tank, that turned out to be tiny pieces of limestone in the gravel I'd used, it was fish friendly gravel too.
Depending on how high the pH is and what fish are in the tank, I would leave things as they if the fish are happy.
Hope that helps.
