Kick Ich and most of the medications like it don't actually work. Those companies are basically just praying on people desperate to help their fish by selling snakeoil unfortunately.
Since ich has been introduced to the display tank, it means that the only way to ensure it is killed off is to put all fish in the tank through hyposalinity. Copper treatment also kills ich and is usually a decent choice, but it's not a good idea to go that route on a puffer because the increased mucus production copper causes on them can give them respiratory problems.
Do you have any live rock or inverts in your 75gallon?
What you'll need to do is lower the salinity of the water your fish are in to 1.009 for about 6 weeks. The fish will do fine at this salinity, but the ich parasite will not survive it. Unfortunately, neither will any invertebrates or the live rock. To kill ich present on the rock, you merely have to put it in an environment with no fish for 6 weeks and the parasites present will die out when they fail to find a fish/host.
There are lots of good articles in this forum on doing hyposalinity.
The easiest way is to put all of the fish in a quarantine tank with PVC T's, etc for hiding places and run hypo on that for 6 weeks. This way, you can leave the display tank empty and don't have to move the rockwork.
If you don't have a large enough quarantine, you can instead take most/all of the live rock out of the tank and put it in a trashcan with normal salinity saltwater, a heater and a powerhead while you run hypo on the display tank for 6 weeks.
Either way, make sure to read through the info on this disease/treatment board to get a better grasp of all of this before you start.
In the future, if you quarantine all new fish before ever adding them to your main tank, you can avoid infecting your already healthy fish.
Good luck!
-Julian