My fishes aren't growing well?

Do you know what you are feeding them? What is the water temperature that they are within?

Additionally, do you notice any other signs that they may be showing? Staying in one area completely, hiding away, not so colorful/dull colors, or possibly are they not eating as much as they used to (or compared to other species of theirs in other tanks)?

Besides what Avery said, how big is your tank? If you keep them in a smaller tank, they probably won't grow as large. I know it sounds weird, but having them in the right size tank will allow them to grow to the full capacity.

Plus, how long have you had them?

Some fish can grow slower due to the size of the tank. What size do you have them in and are they kept with anything else?

Quote from Avery

Do you know what you are feeding them? What is the water temperature that they are within?

Additionally, do you notice any other signs that they may be showing? Staying in one area completely, hiding away, not so colorful/dull colors, or possibly are they not eating as much as they used to (or compared to other species of theirs in other tanks)?

I just got a fantail goldfish yesterday and I love it so much I don't want it to die so thanks for these signs

Frequent fluctuations in water temperature can also cause the fishes to not grow to their full potential.

I believe the fish food and the tank size plays an important role in the fishes growth. Provide nutritious food for your fish. And sometimes, the fish tank might be long, but the width of the tank would be small. That might affect your fishes growth. Have a tank that is long and also has a good width, so that the fish can easily turn around. I think these two suggestions would be helpful.