I have never been a fan of Hypo-salinity ,I think it stresses ,stressed fish more and makes them more susceptible to harm. That said..
Formalin is now known as Formalin-3 it can usually be picked up at any good pet store or ordered online.. It is a pre mixed, it is a 37% solution of Formaldehyde.
This is a powerful oxidizer/chemical, when you use this considerably increase your aeration of the tank/water. Formaldehyde will deplete oxygen in your tank water, it scavenges Oxygen in a big way.
. It is best to use gloves when you are dosing also, granted it is diluted , but it still needs to be respected.
I would not treat a reef with this, because of the same type of things that copper will do. It will remain in residency within the live rock and sand.
If you are going to use Formalin-3, I would use it only in bare bottom QT..
When I had my cichlid business I would use it in my QT tanks when I would get in a shipment of wild caught African cichlids, it is a wonderful tool to have in the arsenal of fishkeeping.
If you want to use copper, you can try Coppersafe by Mardel Labs, it is a chelated copper and (relatively speaking) safer than regular copper sulfate
Cupramine is ionically balanced which is safer than chelate copper and regular copper sulfate,Cuprimine is an active chemically even at lower rates than chelated or plain copper sulfate.
You can use Poly-filter pads by Bio-Marine to remove the copper when you want to from the QT tank or Cuprisorb, I believe Seachem calls it.
I agree wiping your tank down with Vinegar should be sufficient to clean up the tank and safer chemically also. just my 2 cents