What does it mean when looks like a string is coming out the end of goldfish?

This is a bacterial infection that can affect any fish species with prominent fins, particularly Goldfish. It causes the caudal fin to rot, which results in a stringy appearance at the end of the fin. The affected fish may also have difficulty swimming and may show signs of stress. Treatment involves the use of antibiotics and improving water quality.

Fungal Infection

A fungal infection can also cause the appearance of a string coming out of the end of a Goldfish's body. Fungal infections are common in fish that are kept in poor water quality or that are stressed. The affected fish may also have difficulty swimming and may show signs of lethargy. Treatment involves the use of antifungal medications and improving water quality.