[Myka]

Woah, don't mess with pH directly like that, you're just asking for (potentially really big) problems. Directly altering pH won't last long anyway, only a few hrs.

How are you testing pH? If you're using a typical pH test kit then your readings are unlikely to be particularly accurate (and definitely not accurate enough to be messing with vinegar or soda water). If you are using a digital pH meter (good!) then check the calibration as I find some meters lose calibration 2-3 times per year. I have a hard time believing your pH is at 8.7.

Also, alkalinity at 11 dKH is not overly high, some people keep their reefs this high all the time. You do need to raise calcium though, and as you raise the calcium the alkalinity will drop. If you do it slow enough you won't cause precipitation at only 11 dKH. If you raise calcium up to 420 ppm over the next 2 days this should solve your low calcium problem and your high alkalinity problem without a hitch.

Sometimes, causing precipitation is a good way to lower calcium and alkalinity. Awhile back I accidentally hit the "random" button on the timer for my doser. I didn't notice I hit that button and it was like that for 2 days before I noticed the tank "looked funny". So I tested everything and alkalinity was 18 dKH. Since the tank was at this elevated alkalinity for a short time I added a bunch of calcium to precipitate it all out of the water. This worked well, and the result was calcium 440 ppm and alkalinity 11 dKH which was much better. The fish and corals did not mind this because the tank had the high alkalinity just for a short time. This did not ruin any of my pumps. Everything had a white dusting (including the glass lol), but a waterchange afterwards, and a quick rinse of the pumps (it wasn't hardened on the pumps, it came off easy) and all was good.