I was generalizing a bit, my water out of the tap isn't always 8.4, sometimes it's lower, and the water in the tank isn't always mid 7's, sometimes it's 8.0. I don't really check it very often anymore, as there isn't much I can do to it to change it much without risking a wild swing. I feel that as long as any changes aren't drastic, the occupants of the tank will deal with it. Ammonia and such are a different story.
I do have a piece of Mopani driftwood in my tank, but otherwise, I'm not sure why the tank is lower in pH than my tap water, besides maybe by having a couple plants that absorb calcium from the water in the tank.
You might not even need to add CO2 with your light. The more light, the more hassle. As long as the plants are growing slowly without dying off, it's easy. If the plants are melting or dying off like mine did last October, I found a potassium and iron fertilizer and some liquid CO2 turned things around. The fish food supplies the rest of the ferts. It really all starts with the light, but you should be low light with yours, maybe even too low. Read all you can written by Hoppy and Diana, my experience using their advice is very positve.
Wouldn't you like to beside the seaside?