Goldfish are not particularly aggresive, thus combining sizes is not often a problem. In the wild, goldfish can be found in slow-moving, freshwater bodies of water. As with their close relative the carp, they thrive in slightely sludgy water. In an aquarium, bi-weekly water changes are a good idea as a goldfish tank is hard to keep clean. An aquarium with a dirt bottom is ideal but difficult to maintain.
Small pebbles are a suitable substitute for the pond-like bottom.
Typically, goldfish will survive in water temperatures ranging from freezing to 30 degrees centegrade. Goldfish prefer a pH range of 6.5-8.5. Millions of fish are bred each yesr and sold to aquarium shops for resail to fish enthusiasts. In North America there is a demand for goldfish to be used as bait by anglers. Reference written by Robin Street, Undergraduate Research Opportunities Program.