Why does my goldfish have faeces trailing from its bottom?
This is because your fish is not getting enough roughage in its diet. Goldfish and all herbivorous fish and animals need a lot of roughage in their food to ensure their digestive system works properly.
Feed a good quality food such as FishScience Goldfish Flakes which contain optimal levels of roughage. Feeding Daphnia is also a good way to quickly overcome such problems.