I have a 150 gal SW tank. Over the last two weeks the pH has been increasing. I usually keep it between 8.0 - 8.2. It began to increase to 8.4 to 8.5, over the last week and yesterday it got up to 8.8. I did a water change and got it back down to 8.4. The last time i added marine buffer was 3 weeks ago, and I haven't added any since, because it has stayed high. I have not added anything new to the tank since, last fall.Does anyone have any idea what might cause this increase?

with algae consuming the carbon dioxide pH will rise.

I recommend you check out the diy two part to get calcium/alk/magnesium in line.

and don't add buffer based on pH.

what you do (and this is one leg of the cal/alk/mag diy two part) is measure carbonate hardness (KH) or in marine terms alkalinity (alk). Then add small amounts of baking soda to get it to 8-10 degrees. On the overall system you maintain 400 ppm calcium 8-10 dkh and 1350ppm magnesium using common inexpensive bulk chemicals.

so if you research that method I think you will find pH just gets in line.

my .02

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fw leiden since 1979, fo salt since 1979, mixed reef 55g 2002-2009. Strong emphasis on the tank taking care of itself. Balanced with plant life, no water changes, tap water, no filters in FW. Only dosing calcium, alk, mag in marine reef tanks. http://www.aquariumforum.com/f15/my-...ods-26410.html
recent tanks (till 2009) 7 years- 10g FW leiden 7 yrs, 55g mixed reef 7, 2 yrs, 20g FW leiden, 10 g fw leiden , 29g mixed reef, current tank 55g leiden

SW mixes always make a ph of around 8.2 if mixed right. Something seems to be wrong with your ph buffering system. Start making water changes with a new salt water mix. Make a 10% change of your water every few days. Don't use your old Saltwater powder, get a different brand. My local Fish Shop sells pre-mixed Saltwater for $1.50 a gallon. A ph of 8.8 is very dangerous and will kill fish.

Here is a good read, should help you out in more than just your PH, being as its not only your PH that you should be concerned about.
Chemistry And The Aquarium: Solutions To pH Problems — Advanced Aquarist | Aquarist Magazine and Blog
Calcium, KH, GH, pH, Electrolytes & Magnesium in Aquariums; Mineral Ions, Cations

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I don't use mixes. I used filtered seawater from the EPA.