It sounds like you are cycled.
Once you do the water change that you should do after you finish cycling I bet the ph will go up a bit. Do you have circulation on the top of the water, like from a powerhead or return pump? water surface Agitation can help raise ph levels.
But sometimes a lower ph is fine too. i wouldn't go crazy trying to fix it. If it's stable and close to 8.0 I think you're ok. For example, my pH on my 125g reef is CONSTANTLY 7.7 in the AM and close to 8.0 by lights out but it never falters and is always the same so I leave it be.