Only use the high temperature treatment if your fish can handle it. An alternative treatment is medications like the ones found in aquarium stores. Get one for 'fungal infections' and follow the directions. We have no knowledge if the fungal medications are bad for eels. Nematodes - Tiny worms that live inside the eel and borrow their way out through the skin.
All eels have nematodes inside, but the nematodes only burrow out when the eel is unhealthy.
Many eels are weakened by their transport to the pet store, and the stress of moving into your tank. Once the nematodes have decided to burrow out of the eel, there is little you can do, and the eel will most likely die. To prevent nematodes, keep your eel healthy. The eel floats at the top of the tank, and can't seem to get down. This is usually caused by gas, and will go away in a couple days.