I really like the SeaChem Marine Buffer. It buffs your pH to 8.3, and even if you overdose it by accident, it will not raise the pH any higher than 8.3. It is just as good as the Kent buffer (which buffs your pH to somewhere between 8.1 and 8.3).
I've used both, but I've found that I like the SeaChem stuff better. My pH has not changed since I have been using it.