Did you check the PH of the water in the sump before pumping it into the main tank?
What was the PH of the water in the main tank before you added the new water?
A PH of 8 is OK, but, if the water in the main tank was higher than 8, say 8.3 and you pumped in a bunch of water with a lower PH, say 7.8 from the sump, that may have dropped the PH too fast.
Even thought a PH of 8 is still good, it is the speed of the PH change that corals and fish don't like.
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