Given consistant food,temp and clean water regularly, a common goldfish will grow to 10-12". You can actually adjust the rate and size by diet and temp adjustments. Now, in the wild, a fish in the rain forests of Africa or South America near good food and clean water will tend to be larger than a captive bred/raised of the same species. However, if you move to the States, particularly four-season states, food and water temps dont remain a steady constant, the fish will grow smaller than their captive bred brothers.i.e, lake and brook trout, farm raised vs. captive. I knew a guy that had a 30lb lake trout in 500g tank he kept at 75deg, year round. I have heard of fish that big in the wild, but generally speaking, a naturally occuring fish will tend to be smaller than a "stocked" fish. IMHO.
