1. this is heretical, but various people have looked at test kits, and there is not much difference between them when done accurately, i.e. following exact instructions, and IMHO, its a personal decision. I personally found Hanna test kits to be difficult to use and relatively more expensive. The results they give look impressive because they are 'digital' but besides the copper test kit, I don't see them as being a better at measuring ammonia than others (including API). I know this isn't a test kit debate session, so - I'm giving my experience, and anyone is free to use or not use whatever they like.
2. The ring lesion is odd - are you sure that there are no other patches of damage - I would agree with Kanaplex (or Neoplex) in a separate bare tank with a couple hiding places. Adding cycling bacteria may not be as effective in a tank being used with an antibiotic, and water changes may be a better idea. Is the other side of the fact the same? Is it worsening or getting better? Does the fish have any other symptoms.
3. The fact that you have lost many? fish in a 5 day period if I'm reading correctly suggests something else may be going on in this new of.a tank. Please post the results of your parameters not just that they are 'ok'. also - have you used any other medications that may be interacting/.Where did you get the rock, tank, etc.
4. As others have said - a separate treatment tank for nearly every medicine/cure is best, as it rules out problems like this where its unclear whether something is adsorbed onto the rick, and leaching.
5. I'm not a huge fan of dip (again personal choice). One thing you need to be careful of with H2O2 is it degrades (or can degrade) fairly quickly if not stored in a cool, tightly closed container.
6. As to the diagnosis, more pictures would be helpful. Also more information. What fish have died, how did you cycle the tank, again, the parameters. I would suggest the IF you're losing fish regularly after 5 days, its time to stop, regroup, and assess what's going on - and more information from you would helpful, losing 1 fish is a problem losing several means 'stop'. Puffers are not exactly the most hardy fish. As to this being a bacterial infection that developed 'overnight'. This is also somewhat odd, unless throughout the rest of the day, the fish kept getting worse and worse, If this is not the case, it's unclear why it just 'stopped'. I wonder if it's a stress coloration specific to this fish, a viral disease, or even a more common disease presenting in an odd way.
7; General advice. Again, please stop adding fish, especially fish that are somewhat delicate. Make sure that the source of your fish is reliable, does not store their fish in subtheraputic copper and does not routinely make sure that the parameters in your tank (whether a qt tank or display) match as close as possible to the source tank water.
8. To finally answer your original question. I would not use this tank with copper, but rather an empty tank without dead coral/rock. Use dimmed lighting and PVC pipe if you need hiding paces. That said your tank can. 'withstand' copper, but why are you adding it, etc etc.
9. As far as colorine bleach remaining on rock, etc. After using it, always rinse it well then add it to a tank with 2-3 times the recommended dose, rinse again, and id you're s5ill smelling bleach, repeat the process. Use a Chlorine test kit (from a pool test kit, the result should be near 0.
