It sounds like you've got reasonably hard water. Did you let the tap water sit awhile or aerate before testing it, btw? The pH coming out of the tap can be a little off if it's got dissolved gasses that are going to come out after it sits for a bit.
Typical wisdom for tangs seems to be "if it's reasonably close, don't muck with it" since having stable water parameters is more critical than exactly matching the lake's water parameters. I had the good fortune to have rock-hard water when I was out in california, now that I'm back in georgia I'll be playing the water conditioning game .
What kind of hardness is 7dh? Carbonate or general? Your pH is fine, as long as it's buffered to stay there. Check your tank pH right before a water change and see how much difference there is. Organic acids can drop pH if there's not sufficient buffering. If you pH is stable then I wouldn't worry about it. :thumb:
If your pH in the tank is lower, say below 7.4, then consider adding baking soda or epsom salts with water changes. It's not rocket science, and can be done safely without exact measuring. I use about a tablespoon of each with a 20 gallon water change for my tanks. Now, your tap water may require less or more to get the buffering you are shooting for.
A word on buffering substrates: after the initial dust and easily accessible surface molecules are used, sand and coral do not appreciably affect the pH or buffering. Unless you are adding acidic low-ionic strength water, the substrate will not dissolve fast enough to accomodate regular water changes.