I think it was the controller as after a power cycle it read the correct temp. I'm sure the temp was 18.2 last night when I fed them
One of the fish was the CAE so I suspect that they don't like temps that cold, the mullet as a warmer water fish, and for the others I suspect that it was the quick drop in temp - 15 deg in about 15 hours.
Have had a 150w heater in there since lunchtime and the temp has lifted to ~9.5 deg. Have taken it out now and will let the temp continue more slowly. Everything else is looking better now, but will have to watch for disease in the next week or two.