I'm probably repeating myself, but the important part of getting a clunk tank set up is to test it on the ground. On or off the plane, hold the tank so that the outside surface (the surface that points to the outside of the circle) is down. Then tilt the tank lengthwise. You should be able to see where the clunk is, and you should be able to see that it is (a) riding on that outside surface, and (b) smoothly riding to both rear corners as you tilt the tank. If you don't see that, figure out what's wrong and fix it. Sometimes it's stiff tubing, but sometimes it's the various other problems mentioned (I often start by cutting the tubing a bit long, so the clunk hangs up on the rear edge of the tank, and must be shortened).