I'd be dubious about adding a ceph to the system at all.
I work in a public aquarium quarentine unit and we have totally seperate systems for inverts and other fish that can't tollerate copper (Sea Dragons are a good example) and for fish that can as copper is a pretty good cure for cryptocarion.
Copper will remain in substrate as well so make sure you keep on flushing it out.
From a different point is there anything you can use that will bind with the copper and make it less toxic? We make up copper with citric acid but surely there must be ways to move it and bind it somewhere else which might make it easier to remove... I feel some research comming on!
Brittlestars are fairly good indicators of formalin in a system (Which we also use to treat fish, but kills inverts), if there is any then they will die off pretty quickly,not tried them with copper but you could do.
A German company called Merck do our copper tests, they aint cheap but are pretty good.
Hope that helps