I have used Paraguard before.
It is almost reef safe, Dose it slowly every days and observe the coral and the fishes. Stop dosing once you see that the coral having too much stress and fishes start to recover. Sometimes, the coral will not open up during the treatment. It is ok. Once the medicine is removed, the coral will open up. Stop the skimmer as it will skim out the medicine.
Once the fishes is recovered, turn on the skimmer, you will start to collect the medicine in the skimmer container and do water change too. The coral will start to recover.
It is less effective as compared with copper. But if you are keeping coral and cannot catch the fishes out to the QT, it is one of the medicine to use.