I have used Seachem Cupramine for MANY years with puffers.  Follow the directions on the bottle.  Make sure to test the copper levels daily with the appropriate test kit for this type of copper - seachem, red sea, salifert.  Also monitor the pH, ammonia and nitrite.   Treat in a bare bottom tank with NO porous materials - rock, substrate.  Make sure to have adequate aeration/water flow.

The most common reasons for most hobbyists' "failure" with copper are:

1. Treatment is started too late.

2. The copper is administered incorrectly and levels are not monitored.

3.  Water parameters are out of range either due to inadequate filtration or die off of microfauna from rock, substrate.

4. Treatment is done in a tank with carbon, substrate and rock - inadequate levels of copper and die off of microfauna occur.

Please keep us updated.  Best of luck.