PH rollercoaster

[hobbes]

I have had my tank for a few months now. I can never seem to keep my ph level. It goes up then comes down. Same with my alk. I have since last month switched over to RO water and Oceanic salt as opposed to tap water and coralife salt. I have also added a powerhead to my tank. It does seem to be a bit more stable than before but still not constant. I do 5 gallon water changes normally every week.
Am I suppose to buffer everytime I do water change?

 

[barry cuda]

I'd try to resist the temptation to add chemicals of any kind unless absolutely necessary. At a bare minimum, you should test for anything you're putting in, so if you buffer, you need to test for alk and Ca.
*Some* fluctuation in pH is normal. Over the course of the day, with lights on, photosynthesis (from algae) will replace CO2 in the water with oxygen and raise the pH. Then overnight the reverse takes place the pH drops a bit, only to start the whole thing over again when the lights go on in the morning. Are you testing pH at the same time every day? That's the only way to get reliable pH numbers. If it's still swinging up and down I'd look for other problems with the water chemistry.