The Matsya, or Golden Fish, is one of the Ashtamangala, the eight auspicious symbols of the Dharmic religions. This symbol is thought to originally represent the Ganges and Yamuna, the two holy rivers of India. They have come to represent prosperity and good fortune in most beliefs, but in Buddhism they also represent the protection the Dharma offers to Buddhists, who no longer have to fear the sea of sorrows but can be reborn at will, in their choice of forms.
This symbol is interesting because it clearly demonstrates the concept known as transference in anthropology, where a symbol, concept, or belief is absorbed into another culture and given a flavor of its own. In this case the fish symbol was adopted by Buddhism and given a thin cloak of rationalization, and yet is often a key part of modern Buddhism. It is reminiscent of the Christianization of many of the trappings of Mithraism.
In china this is a symbol of fidelity, so items bearing this symbol are considered especially favored as a wedding gift.