Goldfish can lose their scales if the quality of the tank water is not optimal for their health, if they receive a burn, if they scrape against something while swimming or if they are fighting with other fish in their tank. Scales on a goldfish will grow back if the problem causing the symptoms is fixed, according to FishChannel.
Sometimes it is natural for a goldfish to lose scales from swimming through tight tank decorations or from brushing up against a surface the wrong way. If this is the case, the scales will grow back and the fish will not be hurt in the long term. Furthermore, if there are multiple fish in a tank, the fish may fight with each other, which could cause loss of scales. Make sure that there are no territorial fish in the same tank as the goldfish. If a fish is continually purposely rubbing up against a surface, it could be a sign that the fish is infected with a parasite.
Fish can also have skin irritations due to imbalances of chemicals in the tank. Make sure that the tank is not too high in ammonia, as ammonia can burn the fish. A tank's pH levels should remain around 7, which is the neutral point for water.