A temperature fluctuation like that, IMO, is totally preventable. Get a heater, set it to 80 or so so it doesn't drop, and just run a plain old fan over the lights and surface of the water. Keeps it very stable. Somewhere in the 78-82 range is fine, just don't let it move around so much. It is when the tank temps are in the high eighties that chillers become an issue. Lower values are cheaply and very easily stabilized. There is no reason to experience them.
IMO, the only way to eliminate the parasite from the tank is to remove the fish to QT, treat properly with hypo, leave no fish in the main display during that time...from that point on ANYTHING going into the tank must go through QT. All fish must go through full hypo, and all rock, sand and inverts should go into (NON HYPO) QT. This is what many advocate. That is the only true way to fully rid the system. And this is not particularly - if even remotely- realistic, IMO.
I do encourage people to find any possible causes of stress in the tank. IMO, there are a lot of fish, and from the sound of it inappropriate fish, in a 36g tank. At least 3 fish have died (I am assuming the royal gramma is one of them, and a damsel) leaving 2 fish including a tang. So there were at least 5 fish in this tank. I would check for stray voltage, I would list water quality to be sure you are not missing anything, I would list all fish and diets to be sure nothing is missing, and age of tank/filtration, etc.
The thing I always wonder about ick is this, and it is just bugging me-
Many, many fish in LFS systems are exposed to ick from other fish...maybe they even get a few spots in the days after arrival. They are usually not kept in full copper treatment or proper hypo or in QT. They go home to tanks. And yet every fish in every tank is not wiped out from ick...also ick can pop up in tanks that have not had a fish added in a long time... and it seems to me it should be if other fish can just "get" ick from another fish without some stress involved that impacts their immune system. So I think for most hobbyists, ick is present in their tank in some undetectable infection level and it is some stress that weakens the fish and makes in susceptible to broad infection. So while you have to treat the fish, you also have to look for what might be stressing it.