There is no one perfect fish. For beginners goldfish are viewed as easy and normally this is true but this is not necessarily always true. Some types of goldfish for example are not as robust as others and as a result do not survive large fluctuations in water properties. Typically the more ornamental the goldfish is the more prone it is to diseases. Sometimes they will survive the fluctuations but will be weakened making them more prone to disease. However, with proper care and monitoring of water quality your fish should live a long happy lifetime.