Try this....

take the tank down to the LHS and find the K & S brass tubing selection rack. Locate the closest size tubing that just slips over the tubing you have in your tank. Buy that peice and take it home. At home cut off a small peice, say a 1/4" and solder it onto the lines comming out of the tank.

voila' instant barb. WILL NOT pull off with a clamp.