well iv always herd..and been told 20 gallons minimum for one..and 10 for every one after that..so 4 goldies..if they were all fancy would be about 40 gallons....but you have a Subukin in there ..witch needs more room to swim...(though your tank is long)..so i would say they would be happier in a 50-55 gallon tank.. The Subukin is a pond fish and will get way bigger then your fancies and will out swim them, making them get less food.By what measure? The water chemistry parameters are perfect.
not flaming here your tank and fish are lovely!...but just saying