Most likely it is either a water quality issue or the fish are simply changing color. Goldfish require huge tanks as they produce a lot of waste, so 4 juvenile goldfish would need at least a 55 gallon tank initially, to prevent an ammonia or nitrite spike. We would recommend testing pH, ammonia, and nitrite. Change about 30% of the water and add AquaSafe Plus as well as SafeStart Plus to the new water.
Goldfish change color at least once during the first year of their lives, and may change again later on. If the water quality is good, they are eating well, there is no gasping, darting wildly about, rubbing against objects, torn fins, bloody areas, white spots, etc., then they may simply be changing at this time. I suspect this is not the case, because it usually does not happen at the same time for several fish at once.
Date published: 2017-01-27