You are definitely not alone with this problem, and Aprilia is not the only manufacturer affected. We have heard of many instances of plastic tanks expanding, and have even seen it in our shop here. The problem led to a class-action lawsuit against Ducati North America, which was settled earlier this year with an extended warranty on certain Ducati models with plastic tanks. The expansion seems to be a result of ethanol used in the fuel; ethanol can absorb water, which is then absorbed by certain types of plastic used in making fuel tanks, resulting in expansion. The ideal solution once you get a new tank is to avoid ethanol-enhanced fuels, which you will not be able to do easily — all commercially available gasoline in California contains between 5.7 and 10 percent ethanol, and there are no labeling requirements for pumps so it will be difficult to know just how much ethanol you are putting in your tank. Ethanol is also causing trouble in the marine and small-engine industries, so there is ethanol-free available fuel if you hunt around. One option is to use an additive such as Sta-Bil Ethanol Treatment, which is said to help remove water from fuel. Another is to seal the tank with an ethanol-resistant epoxy, such as Caswell Gas Tank Sealer. If you store your bike for any period of time (doubtful if you live in California), leave the tank empty or fill it with ethanol-free fuel.