Its quite concerning that you put copper in your fish tank. That would have a major negative impact on a tank with live rock and sand. You do not have a fish only tank if you have live rock. Live rock is live because of the invertebrate marine organisms and algae living in the rock. I couldn't find an ingredients list for Pointex other than vague references to natural ingredients, but you mentioned copper. If you have a list of ingredients, please let us know.
Can you post up all of your water readings: ammonia, nitrite, nitrate. This is important. What do you use for your source water? How often are you doing water changes?
As to stray voltage, you have had this fish for years wo problems and now you have a problem due to the treatments last year. IMO the best course of action is to work toward bringing the environment of your aqua back to pre-treatment conditions, if possible. Copper tends to linger in aquariums becoming absorbed in to the rocks, the glass seals, plastic/rubber equipment, etc.
Begin using Selcon by American Marine, Inc. This will help address the HLLE.
https://americanmarineusa.com/pages/selcon
I'd recommend doing daily or every other day water changes, 2 gals a day, using RO or DI water (test it to insure no nitrates unless you have your own
marine RODI system). Continue doing this until the fish improves. If you have high nitrates, then we'll need to discuss that problem and what to do.
You need a QT if you plan to add more fish. The likely result of not having one can is loosing your fish and possibly even your whole system. A 30 gal system should work fine. If fish are properly QT-ed before adding them in your tank, you will not find yourself in a situation where you need to QT all your fish due to a disease you've introduced into your system. QT prevents diseases in the display tank.