How Old is your Gold Fish?

My Spongebob has been alone his whole life, poor little baby, he keeps out growing all his tanks so every time he grows a little bit we buy a new tank for him. I have heard people say that a goldfish will only grow as big as it's tank. I left my gold fish under my mum's care when I left home, so she is now in charge of the cleaning, feeding and everything else. The funny thing is my sister has gold fish too, we bought them at the same time, from the same petstore, and my sisters fish died within a few months of getting home! Since then she has been through 5 -6 fish, while I have just had the one!