Golden fish eggs are items that can be gained whilst training Fishing, looting the Motherlode Maw, opening golden prawn crackers, prawn balls or prawn crackers, at a rate of 1/200. They can be gained without the requisite level in Fishing as golden mystery eggs if it is gained through golden prawn crackers, prawn balls or prawn crackers.
It is not possible to gain a second egg of the same fish. At level 99 Fishing, it averages two to three hours per golden fish egg. One golden mystery egg is a guaranteed reward as the Tier 2 Prawn Perk from the Prawnbroker. A Golden prawn cracker, which grants both you and the person you use it on a golden mystery egg, is a guaranteed reward as the tier 4 Prawn Perk.
It is not required to have an aquarium built to obtain golden fish eggs.