This could be an indication of toxins in the water. Change 20% of the water monthly. Fish waste and uneaten food build up chemicals that can make your fish unhealthy.
Your fish may have the desease known as ick. It is common for pet fish to catch. It can be cured by buying the medicine at any local pet shop. You only need to add a few drops into the tank on a daily basis until the fish seem healthy again.
Your fish needs to be taken out of the tank and you need to take the levels of the water and make sure everything is normal. Its seems that your fish my be sick. There are Vets who will examine the little guy and let you know what is really going on.
