I think most members on this forum recommend a 20 gallon for 1 goldfish + 10 gallon for each additional fish . Goldfish, even the fancy varieties , will grow to at least 6 inches in length so a bigger size aquarium has more water volume to dilute the wastes generated by the fish, which means less water changes and work for you.  Also you will need a good quality, high volume filter because goldfish are messy. They are a cold water fish, so depending on how cold your house is and where you live you might not need the heater. You can feed them 2 to 3 times a day the amount of food they can consume in 3 to 4 minutes. Goldfish flakes and pellets should be the main part of their diet, but you can also feed them tiny pieces of watermelon, oranges, and peas for additional vitamins and nutrients.

If you intend to keep goldfish in an outside , make sure to put a wire mesh over the pond or fish container if you have stray cats and other wild animals around.

ooops forget about the fruits/veggies part. hehe sorry i didnt finish reading your post before replying