do you buffer your top off water (either with pH buffer or kalkwasser)?
Otherwise of course regular tap water will be around neutral (7) pH.
Food being metabolized in the tank also contributes to it getting lower over time.
Completely normal
If you replace the evaporated water with kalkwasser the pH will be high in the make up water anyway. If you don't use kalk you could simply buffer your freshwater. Reef Buffer by seachem will work better than Marine Buffer if you have a lot of corals/inverts to feed.
What's your Alkalinity though? If your alkalinity is high but your pH is low you may have to find a way to get O2 in and CO2 out.
Here's a very informative link:
http://www.reefkeeping.com/issues/2004-09/rhf/index.php