Same risk as with any lion exposed to copper, so you never know. Did you run the full course of prazi, make sure to check for signs of internal parasites, stingy poo(hanging or even around the tank). Metroplex treats another form of parasites that prazi doesn't, and all lions have a high risk of internal parasites. Sometimes you need general cure in the water column, which contains both prazi and metro, internal parasites can be quite stubborn sometimes.
Going off feeding may also be him preparing to shed, his color may look faded and eyes even cloudy, give it a few days and he may shed then be back to normal. This is not unusual going into a new tank and being treated.
How much has eaten so far.