If you stirred the sand, it's possible that you released hydrogen sulfate into the water column. There is usually an odor associated with the release, but if you weren't there long after stirring, and stayed away long enough for it to be released into the atmosphere, you may not have noticed it. Hydrogen sulfate is very toxic to fish, and if it that was indeed the case, you're lucky you didn't lose all your livestock.
As for your pH, the time of day it's tested will yield different results. PH will be low in the early morning, and rise as the day goes on. It's highest at lights out, and starts to fall after that. Taking a sample to the LFS is okay, if you get there quickly. If it sits in the container for a long period of time, the pH level will be different than what it was when collected. Low pH is not normally a fish killer. Broad pH swings are fish killers. With 11 dKH, your pH should be running quite a bit higher, but it's not what I would consider critically low. I use Reef Crystals, and dose Alkalinity as well. My dKH is normally around 9 dKH, and my pH is usually around 7.9 early morning. I don't focus too much on hitting a specific number with pH. What's more important is stability. Marine animals can adapt to different pH levels, but they can't tolerate broad swings in pH, especially if it happens quickly. Unless you have a lot of corals consuming a lot of elements, do not use Sea Chem Marine Buffer. Yes, it raises pH, but also alkalinity, calcium, magnesium, strontium, potassium, and other elements that may not need supplementing. Unless you have more than soft corals in your tank, you should be able to maintain adequate levels of all elements with regular water changes. Never add supplements of any kind without testing for them first.
