A 3 degree temp swing isn't going to kill a fish; especially a species as hardy as a clownfish. I also quite honestly doubt that ich can kill a clown. Only thing two things I can think of would be Brooklynella or possibly Marine Velvet disease. Was this a newly acquired clownfish? They are very susceptible to brook.
I've lost four in the past few months after keeping them successfully for years. I'm starting to think the LFS I'm buying them from may be the problem. Does Brook stay alive in the water like ich does?