Can fish feel pain when they have fin rot?

Fin rot while it is on the fins doesn't really harm the fish too much. The point is, you have to stop it when it starts, because like a vehicle that rusts, the rot will eat its way down to the body.

Imagine if your body started to get eaten away from a flesh eating bacterium. Sores will appear, blood will flow, and like any other living creature, you would die.