Mango's are less common than goldies and are perfect for smaller tanks. Goldies are more attractive IMO when they are small (under 8"). Because Goldies grow pretty fast, when provided with the right care, they need a tank with at least 18" in depth. A few weeks ago the store I manage took in a 14" goldie raised in a 55 gallon. Its tail was bent upright because the tank did not offer enough depth.