Cuprisorb works well. It will also bind lots of different type of metals. Poly will do the same thing. It will regenerate - I have a big bag that you can borrow if you want.
Calcium Carbonate will bond copper - just a normal amount of sand or a piece of LR or two will absorb enough copper to kill ich/velvet/etc which is why you don't see any in QT/hospital tanks. The bond is usually permanent, but if the CaCO3 dissolves, then the copper will get released. ...so, the blue poly reading might not be from copper since most get absorbed by sand or live rock, or you have an excessive amount that would surely mean invert death. In any case, you might need to leave some in the tank for a long time, if copper is the real issue. Typically, a tank that was once used as a copper treatment hospital is safe to use again with a good rinsing because the minimal residual amounts will be quickly bonded by the aragonite. If you have a good amount of sand or real live rock, then I might suggest that you keep looking for whatever is at the root of your issue since I would suppose that there is very little free copper in the water - man made rock and no/silica sand could be an issue.
Seth is right - Kent salt will oftentimes turn a poly filter blue on it's own. No bueno for a reef, IMO, but lots of people use it still with what they think is good success, until they switch brands and typically watch stuff take a step forward. I don't know what is in Kent, but it will turn a poly filter blue where other salts will not. If you aren't using Kent, then sorry for the sentences.
