When my kids were younger, we had a 2 gallon tank with 3 goldfish. They always were very active. When it was feeding time, I sprinkled some food on the water. It was gone within seconds!!
Now we have a large outdoor pond. We notoced that goldfish grow much larger in a pond!! It is amazing! My daughter won a little goldfish and put it in a small tank first. Two weeks later she moved it to the pond. You should see that fish now: it tripled its size ; We even found that he "made" some "offspring"