did you make the frag mounts yourself? or were they boston aqua farm ones? i have the BAF frag plugs, and on the package it says not to add in large batches, b/c even though they "claim they are cured", they will significantly mess up your pH and alkalinity if you add too many too quick, in small system.
for my 50g system, i started adding frag plugs about 1-2 every 2 days or so just to be on the safe side. i was being way overly cautious and probably could have added more, faster.
the fact that you never saw any NO3, NO2, and NH3, would lead me to guess that it was caused by dramatic shifts in pH and alkalinity. did you happen to test for these?
if you made the plugs, its possible there was some other contaminant also....silicates, phosphates etc.
i dont see how just adding a sump would cause problems. i've added tanks and large new volumes of saltwater to my system and other people's systems with no problems. only issue i ever saw was b/c of all the new saltwater (even though the same % on the refractometer), everything would slime up quite a bit, and some of the sps would get ticked and turn brown over the next couple days.
i think i saw another pic somewhere on zoosrus one time.
