This is the black flexible stuff. It's the best clunk line out there. Just make sure to divide the clunk line in two by putting a short piece of brass tubing in the middle. That's because this stuff is so flexible it could end up stuck at the front of the tank if you use a really heavy clunk (most people use the large one on these tank setups). If you put the extra piece of brass tubing in the middle it will be fine.

http://www.ronlund.com/Merchant2/mer...ct_Code=HAY249