I'm not sure how to translate "losse"; loose?
Anyway, getting the tank off requires removal of the side panels along the driver's seat, then the seat itself, then the big bolt/nut combo under the seat, then the bolt at the gauge end of the tank (metric allen IIRC, 7?), then the two side allen bolts that are right on the frame and are colored black. I suspect you didn't note those latter two and thus your problem. They're allen wrench bolts too, but the size escapes me.
The fuel line is a quick disconnect so you'll need some help probably to get it off if you want to be really careful. While you're working on the air filter you'll want to keep the tank in an upright position to minimize gas spillage. I use a 5 gallon bucket that orignially had asphalt tar and the fuel pump and fuel lines sit nicely in the bucket with the tank in its natural position; balances beautifully.