sickness

Question
i have 4 goldfish in a 55 gallon tank 1 of them has a little red spot on its tail its still swimming fine but a little less avtive than the others can you tell me how to cure it

Answer
Andre,
It is hard to tell without seeing a picture. It could have been from flashing and now he has a sore, or possibly scraped himself on something and now has an open wound. Or he could be suffering from poor water quality.
Either way, get your water tested for ammonia/nitrite/nitrate levels and make sure everything is optimum.
With four goldfish you'll want to do about 40-50 percent water changes every week to week and a half. Testing often will help you decide the best cleaning routine and when you should do water changes.
Ammonia and nitrates should be at 0, and nitrates should never exceed 20.
Add some aquarium salt to the tank, and bring the temp to about 77 degrees. That should help if it is indeed a sore.