When you've ruled out the other basic options like parameter swings - definitely start looking at the leathers. It could be that or a parasite. Ive seen in other threads before that the Euphyllia seem to be more sensitive to the toxins from the leathers and tend to bail first.is it common for leathers to cause issues with LPS? I had a die of of a few hammers as well, and wonder if my leathers are the killers.
In this case - I would be suspicious of two items:
1) Nitrates are too high @ 47.6. Something is going on to be that elevated and could cause irritation to the corals - especially the Euphyllia
2) Leathers could be emitting toxins from the anemone stings (highly likely) - however its a question whether the carbon would scrub enough of that out. Its possible its not because the initial drop of toxin into the water will be somewhat circulated prior to getting to the carbon scrubber.