IMO, 7.3 kh, if consistent is ok. But like what Sherman advised 8+ is better.This might be your problem, but remote.
Other ways that have worked for me 1 ) Bring the frag out, gives it a cold shower under sprayed tap water. 3 X 1 min. The zoas will closed up, so its safe. The culprits, whatever they are, will be hopefully flushed off. ( 2 ) If unable to take the frar/rock out, then blow the zoas colony with a wave maker or whatever you have, to send the irritants away. ( 3 ) check for the small irregular shaped small white, greyish star. ( i dont know the name, nvr can remember ). They are zoas eater. Take them off and toss it into the tiolet bowl.No need to flush !( 4 ) If its nudibranchs, consult the pest controllers like DR FuEl.
Good Luck,