Bristleworms can cause skin irritations with their bristles. It is recommended that you use gloves when handling bristleworms. Pick them up with tweezers not your hand. You can buy bristleworm traps at your local pet store. The best prevention is to thoroughly clean your coral and rock before adding it to your aquarium.
All fish have a protective coating to protect their scales or skin from disease and injury.
Salt helps fish to produce this protective coating. Obviously, saltwater fish have salt to help keep their skin healthy. However, live-bearing freshwater fish such as mollies, platys, guppies and goldfish and other brackish water fish will benefit from 1 teaspoon of rock salt per 5 gallons to help them produce their protective coating. It's because fish are territorial and have a vested interest in a particular piece of plant, decoration or space in your aquarium. To combat this problem, before adding new fish, try taking out all the plants and decorations.