I'll be honest, with a tang and 2 puffers, that alone is enough to have elevated nitrates in a 55. If you are reading 0, i'd suspect either a bad/not done correctly test or you have some cyano
or algae growth going on that is giving you a false reading. That is why your anemone is stressed.
As for the light, not familiar with the unit and read about it. Some say it is fine, others say not...supposed to be great for soft and LPS
coral, but no one can agree on SPS
and other things that require high intensity lighting. I think it might be fine if we can get the parameters stabilized.
I'd suggest returning the tang and one, if not both, puffers.