This is a fairly common problem with clunk tanks. The fix is to run a second piece of tubing around the nipple and clunk ends of the line in the tank. The simplest way to do this is with a tool GMK makes called a Silver Bullet. If you don't have access to one you can look it up on Google and the picture will give you a pretty good idea of what to use as a substitute. I have always used snap ring pliers to get the second ring of tubing over the main line . Once you "clamp" this with fuel tubing it should not come apart . Also check the edges of the clunk and make sure there are no sharp edges on the nipple to cut the line.