Sudden and unknown fish deaths?

I understand how things like this can be. I agree with the other person that answered as for the dropping of the temp. 82 is the absolute highest with 77 to 78 best. I know that you say the spray paint is for under water use but it might not be for fish use. Check to see if any of the paint has chipped off. Fish will see it and try to eat it and if they do it can be deadly. Everything else you are doing seems right so it might be the paint. Good luck and I hope it all works out.