Sick goldfish

Question
I have a 50 litre fish tank that is currently holding 6 cold water gold fish. One of the fish appears to be having trouble swimming.She will move a little then go to the top of the tank and floats on her side. A month or 2 ago she was constipated. We stopped food for 4 days then gave greens She appeared to recover but has gone down hill again.
Please can you help me save my fish

Answer
Sounds like the same problem. Fast her for 2 days and on the third day feed her a cook shelled pea then fast again for 2 days. I would also think about moving all the fish to a larger tank. The one they are in is way too small. Also think about feeding food that sinks. When goldfish come to the top to eat flake food, they also swallow air.