I had a bad experience with it, completely my fault - made a dumb decision. I had a "Kalk Stirrer" by Two Little Fishies, with my ato line feeding it and then just gravity fed to the tank. I refilled the kalk and stirred it all up really well, then decided to do a water change. My mistake was not shutting off the ato pump. As the water change was going on the water level in my sump dropped, I failed to see that all of the fresh mixed kalk was pouring into the sump. I still didn't see it when I put the system back on line. My pH shot up over 20 (I think the Apex was reading in the 20's but at some point the hobby grade pH probe has to exceede it's limitations and just reads random gibberish values?) & Alk was through the roof. And I did this at night, once the WC was complete I went to bed. Next day I woke up to an essentially nuked tank. I think a lot of the fish were ok...but none of the corals survived, and this whole thing was the day after Denver MACNA so I had spent a lot of money on corals.
I think in my case it was just a combination of stupidity on my part. If I waited to mix the kalk until after the WC, probably wouldn't have been as bad. It I remembered to shut off ATO, wouldn't have even happened. If I looked in the sump during or after the WC and saw a white blizzard of kalk, I wouldn't have turned on the return pump, crash wouldn't have happened.
I think my story is just a cautionary tale though, the same thing might be able to happen if a dosing pump gets stuck "on" or heater sticks or shorts out. There is probably a million things that *could* go wrong....I'm trying to see how many of them I can check off :laughing::tired: