If you’re looking for leaks get a vacuum cleaner that blows or a lilo inflator and hold it about an inch away from the bung. (don’t push it up tight or you will blow up the tank – you are just trying to increase the pressure inside slightly) Get a mixture of washing up liquid (10%inc) + water and paint it around all the joins including the keel band and rudder fittings. Any leaks will be obvious as bubbles will form. The bubbles will be obvious as they blow up and pop. You will probably find the hatch covers leak!