Try to hit 8.3-9.3 on alk. I've found a lot of corals, and a nem is sort of one, react profoundly to low alk. I use Kent dkh buffer pegged to my evap rate, dissolved in my topoff water, keeps the alk steady. I've found that one reading being out of whack accounts for a lot of tank iffy-ness. Don't know if that will help. Also, on your extreme flow, you might slit a coarse sponge and cover the head with it: if it blows it off, tie it on with fishing line---won't corrode or break. Be sure the sponge is aquarium rated: most kitchen sponges you can buy are treated with antimicrobials. Your lfs might have one they'll deal you: a huge one comes with bigger mag pumps, and a lot of people discard them.