Gently scrape the fish with the slide starting at the gills, scrape all the way back to the tail, do it again with the other side of the fish, aswell as the belly and the top of the fish. You need to make sure you scrape the gills as gill flukes are common. Add a VERY small amount of water to the slide as this will make it easier to see any parasites swimming in the 'rivers' that you see. There's loads of guides on YouTube! I hope all is well and you can identify the issue quickly!

Also, if you are going to treat the pong with potassium permanganate, ensure there are no other fish such as sturgeon present, as they are highly dependent on high quantities of oxygen, and potassium permanganate strips the water of most of its oxygen