If we rule out ammonia (and nitrite), a next likely cause would be toxins. Any cleaners being used around the tank lately? Bug sprays, air freshener, etc.? Have you had your arm in the tank? Soaps, lotions, etc.?
no cleaners, no bug spry, and I rinse my arm well before I stick it in the tank when I need to get in there, and I haven't used any kind of lotions for some time now
Adding a Polyfilter to your tank would help remove and identify (color change) several possible toxins.
I'll have to look for some
What have you been feeding the tank, and how often?
In particular, what were you feeding the Lionfish? How often?
I was feeding 2 times a day brine shrimp or mysis 1 cube each time, with a small pinch of flake for the foxface, and once every other day one silver side or 1 to 1 and a half krill for the lion fish (he also eat mysis when I stuck it in the tank.)
Despite the ammonia reading of zero, I would still do a couple of larger water changes. It's still a prime candidate, and one of the safest steps to take.
I am.
How are the other fish looking and behaving?
same as they always had, eating fine, not looking stressed