I had exactly the same problem - and it was gushing out of mine.
It's 8mm bore petrol hose and it will cost pennies to fix but getting the tank out is not much fun.
Luckily my bolts came out - s-l-o-w-l-y but they did come out.
If they are all spinning then either the captive threads in the chassis are all seperated (unlikely) or they are all broken/loose.
If the latter is true then the tank and carrier should have fallen out by now.
Have you got the right bolts? It's the ones at the sides not front and rear - the ones front and rear hold on the straps which in turn hold the tank down into the metal cage which in turn is held in by 2 bolts either side behind the springs. I had to loosen the exhaust to get at one side by dropping a rubber hanger out of the way but no worries.
Here's my tank out:

This is shot from the rear looking forward.
There is a lip on the left which is the place to look for the 2 left side bolts. And the same setup on the right. You can see a bolt close to you which would be on the rear side of the tank in situ - this is the (broken) strap bolt I talked about - loosening it while the tank is still in the car does ziltch.
One other thing - on my car the 2 right hand side bolts were 17mm and on the left both were 16mm. I loosened the 17mm first and then thought things were odd on the other side til I realised they were smaller for some unknown reason. Worth checking.