No wonder Goldfish have been nicknamed Water Puppies Goldfish are very enjoyable once you have the correct set up for them! Despite beliefs that they can live in fish bowls, fancy goldfish grow to be about 6-8. Common goldfish get even bigger, maxing out to about 10 in aquariums. We highly recommend at least 20 gallons for juvenile goldfish and 35 gallons for a single adult goldfish. If you have more goldfish in the aquarium then go bigger. This is due to how messy goldfish are, they make a lot more waste than some other commonly kept fish and therefore need a larger water volume. Maintenance on your goldfish aquarium is also very important! Since they are messy, we recommend water changes of about 30-50% every week or at the very least every two weeks.