i have a fantail goldfish too, everybody asks why shes so fat. it's just their breed they are bred fat. Females have a even bigger abdomen! I had her in quite a small fish tank. I moved her into a 7 gallon recentley with a filter! I have never seen her happier! Yeah she dosent go near the bubbles either. But shes still happy! And i got her some friends now shes even happier! Good luck with your fantail. They are fat by nature!