Yes, more information is def needed here! How big is your tank? How long has been running with fish? Is it cycled? How many/how big/what type of goldfish are in there? How often do you pwcs/how much? What type of filter? How are you testing your water parameters (amm/nitrite/nitrate)-liquid or strips? What are your water parameter readings (numbers)? What are you using as a water conditioner? It sounds like your sick fish is suffering from ammonia and/or nitrite poisoning. Ammonia & nitrite are lethal to your fish. You need to be testing your water & doing large daily pwcs with a good water conditioner (prime or amquel plus) to keep your amm/nitrite levels below .25. Also, fancy goldfish need 20gal for the 1st fish, 10gal for each fish after (this is the minimum rule)-if you dont have an appropriate sized tank, you will need to consider upgrading your tank or rehoming/returning your fish & getting something more appropriate. More information will help us to help you!
