Hmm, not sure which product you are using some are chelated some not; for now just dose per the instructions and forget the dang test kit. Some of the chelates can be very hard to measure accurately, do not, repeat do not overdose! Most importantly OBSERVE the fish several times a day if possible for signs of stress. Are white spots going away? IME, usually only one or two treatments of copper are enough, you should see results in 24-48 hours. Watch that puffer closely, they do not always respond well to copper, (no scales) I have treated some with good success but you may need to take that one to your LFS if stressed, it would have died from ich anyway if you left it in the display. Plenty of aeration is our friend here; drop an airstone in they are under rated and of much benefit. Change water if ammonia/stress go up. I feel your salinity is good, and do NOT suggest going lower at all, indeed it's more than low enough now in my opinion, I have never dropped it below 1.018 but that's just me. Continue to feed and remove all uneaten food/fecal as soon as it shows. I have a feeling you responded to this problem late so don't expect miracles, but best wishes and at least now you have a QT you can use for future.
