Metronidazole will not do anything against ich. Copper will, but some fish are very sensitive to copper, and if the fish is sick already, copper may stress it enough to kill it.
There are only two methods that kill ich -- copper treatment and hyposalinity. You really need to research hyposalinity before you try it. Dropping the salinity too quickly can also kill your fish. It must be done gradually over several days. I hope you mean you are dropping it down to 1.020 or 1.010. 0.020 or 0.010 is basically fresh water and will kill everything. Hyposalinity treatment should be done at around 1.009 or 1.010. Anything above that will not kill the parasite. You should be using a refractometer (not a hydrometer) to measure the salinity, as hydrometers are not accurate enough for the purposes of hyposalinity treatment.
I hope you did not add copper to your tank, or try lowering the salinity in your tank. If you did, you have killed everything in your tank -- inverts and bacteria cannot survive these treatments. Copper will get into the rocks and sand and destroy them. If you added copper to your tank, if you ever want to keep corals or inverts, you will have to throw away all your rock and sand and start over again. Copper does irreparable damage to rocks and sand. This is why, when you treat for ich, you must catch and remove every fish to a separate quarantine tank. And you have to treat every fish. Not just the ones that show symptoms. Any fish that is in the tank will be carrying the parasite. If you treat one fish, it will just catch it again from the others.
Raising your temperature to 84 isn't doing anything but stressing your fish further. 84 is not going to kill the parasite, and is not warm enough to speed up their life cycle. Keep your temperature normal, at what you usually keep it.