Your light is plenty, and no amo is good, he could still be alive in there somewhere.
I can't remember hearing about any toxins killing fish, mainly amo spikes and funky cloudy water.
He could have a torn foot, or just trying to acclimate.
I'd look real good for him.
In placing nems, (my sebae and LTA) I put at bottom w/sand scooped a bit, and between rocks, basically a little nook.
That way it can back in or out for more/less light or flow.
Also, moderate flow, not blasting it.
Tents should move gently.
Good luck.