I found this point from Seachem page worth repeating
Leave the copper-absorbing media in your tank for at least a week after the copper concentration has reached 0 to ensure all traces of the medication have been completely removed.
Seachem notes the importance of
measuring copper levels in the water column when treating with copper based medications and when removing residual copper at end of treatment
If you don’t have a copper test kit, I’d suggest several large water changes followed by active carbon
Alternate method (re fish disease handbook etc)
75% water change upon discontinuation of copper treatment - this can also be done as two separate 50% water changes on consecutive days (if you don’t usually perform large weekly water changes, this is a more conservative approach)
Add fresh activated carbon (marine grade generally has a “dose” recommendation)
After 24 hours, replace with fresh carbon
After 48 hours, replace with fresh carbon
Leave this third carbon “dose” in place for a week
Note there is no disadvantage beyond time to performing this method over a couple weeks, ie, refresh carbon after 2-3 days etc, the large water changes can also be spaced, just be sure to perform another large water change just before adding carbon
I’d further suggest 50-75% water changes before each new batch of carbon
Copper is very difficult to remove completely from an aquarium containing substrate & decor (which is why it’s recommended to perform copper treatments in a bare hospital tank)
Most fish are fine with residual copper levels
Most shrimp tolerate residual copper (ie after carbon “clean up”)
Some shrimp can be very sensitive to residual copper (eg, wild Sulawesi shrimp)
