Looking at your livestock, I insist you put a camera and watch your fish.
I've seen Sailfin, Fox Face and Melanurus all nipping on coral, some are sneakier than others and would not do this when you around, and may even wait till lights out.
I know you think they are well behaved, and you watched them but you never know until you know.
Here is my very own, recent experience with nipping:
4-5 months ago my 9 year old clam suddenly closed and wouldn't open, we didn't know what was going on, but it kept getting worst until it finally died. 2 months ago we bought 2 new, way-too-much-expensive clams, they started growing instantly and were thriving until a month ago when we spotted severe biting marks on both of them. Not knowing what we dealt with we moved the clams to our sump to let them recover.
A couple of days later, I was looking at another one of new additions, a tiny, even more expensive OG bounce.
It was nearly a week in our system, and it was all teared up.
Shocked, I moved what was left from it to the sump.
We suspected it was our new emerald crab, as this was our latest addition to the system.
Well, we caught him, and brought back our clams.
A few moments later, before we even moved the bounce back we spotted our so far well behaved Melanurus tearing up one clam after another, with so much aggression that if we wouldn't have spooked him away he probably teared them apart completely in less than 10 minutes.
Now mind you, we got both our old clam and Melanurus a few months apart from each other, both were added 9 years ago, and the Melanurus did not show any signs of nipping until this very moment.
I don't know if this is really what's going on in your tank, and I strongly believe the harsh dipping could've been responsible for some of the symptoms, but given the type of fish you have and their size, my suspicion for nipping is high.
