I can't tell you specifically: I'm sps and lps, but I've had leathers in the past.
Check the following: temperature, by more than one thermometer: it's getting hotter, and thermometers like like rugs.
alkalinity: s/b 8.3 to 11
calcium: 400-420
salinity: 1.024-6
If parameters are screwed, do a 20% water change with ro/di saltwater, then re-test.
Run carbon, in an unused nylon stocking if you haven't got a bag.
What unusual thing have you added/done/changed? Did the leathers' unhappiness follow that?