Wow. I just notice this in one of your photos.
To me that looks like a puffer fish. (Dog Face species)
Cool. I always wanted to have a puffer fish....but I can't...(I have corals)
Have you had him for a long time?
They have Big Strong Teeth. (Has he ever bitten you)
They are usually not safe for reef and will eat ALL corals (yet you have LPS corals and still there)
Big fish too.
https://animal-world.com/encyclo/marine/puffers/dogface.php
I don't know much about them, but that link above says they eat a lot. Are messy and need frequenty water changes. So maybe that's why it was recommened to do you large water change.