Hi there,
yes, puffers are very sensitive to copper. HOWEVER, the dogface puffer can go through a copper treatment, BUT not the same concentration of copper as the copper bottle will say. If you do follow those instuctions as per the bottle you will KILL your puffer. I do have a copper dose amounts for puffers ONLY, if your interested. Also I would need to know what kind of copper you were thinking of using?? It does make a difference. BUT a copper treatment should be your LAST resort. Especially if the puffer is still eating and is too new. Here's some questions for ya.
1. How big is your Quarantine tank?
2. How big is the puffer?
3. Are you sure the puffer has ich?
4. Is the puffer breathing heavy?
5. Have you done the freshwater dip test?
It will give me a better idea of the puffers conditions and how fast the water could spoil. With you adding more water from the main tank. It will never flush the ich eggs out and if you know the life cycle of ich, it doesn't hatch or fall off the fish for approx 3-7 days before it re hatches.
If you are SURE, you indeed have ich. I would suggest doing a more natural method I use, if your up for it. Let me know.
Hope this helps.
p.s Food - I feed my puffers Krill & mysis. They seem to like that best. Some times I give them a live snail or live crab for a treat.