How old is your tank?
What time of the day are you testing?
You could have a built-up of CO2 in your water, or even your home. Sometimes this time of year we get all bottled up by having the windows closed and the AC on. Can you open a window? If the environmental CO2 isn't a problem sometimes it has to do with tank aeration and simply adding an airstone or creating more water turbulance can raise the pH. Test the tank, take a cup of that tank water outside and aerate it for a bit and then test that cup of water. Did it go up? Do you have a skimmer? How deep is your tank?
When you do your water changes do you clean the tank? Could there be a buildup of organic matter? Have you ever tried using a pump to blow off your rocks before a water change. Did anything go missing lately? What is the stocking level of your tank? Do you have a heavy bioload? What are your nitrates?
How do you prepare the water for making saltwater? Do you aerate it? Do you buffer it? What salt mix do you use? What is the pH of the new saltwater before you add it to the tank? If you mix your own salt, do you add the salt to the total volume of water or do you pour the water into a bucket that already has the salt in it? Is your water RO/DI or tap or well or ?
Do you use any sort of water conditioner like Amquel?
Ph is a "result" more than an actual test, and your "result" means that something is not optimum. As Cranberry mentioned, co2 is usually the leading cause.
Another is an imbalance of your alk and calcium. Ammonia will pull it down too...
There is no quick answer to fix low ph, it's just an indication that you now have to test other stuff.
Originally Posted by roadie996 http:///forum/post/3102946
I think you need some reef buffer to raise it up instead of doing water changes
It should be done with a lot of caution. This can be very problematic advice if his problem is not caused by low alk. If he is doing lots of water changes, he probably does not have an alk problem but could. Just dosing buffer, without monitoring alk and calcium could be a real problem. As noted above, pH issues can be as simple as having glass tops or poor circulation.