Home depot sells clear, flexible polyethylene tubing for cheap- it'll look something like this: http://www.bio-world.com/MoreInfo.asp?ItemCode=475993
Most LDPE (Low Density PolyEthylene) doesn't have plasticizer in it, and will work great in saltwater. Just pump saltwater through the tubing youre going to use in a bucket first, and check to make sure the tubing doesnt show discoloration or stiffness. Any nasty coatings or unreacted monomer should leach out into the bucket and not your tank. If youre paranoid, look for tubing that says "non-toxic, plasticizer free", and "NSF/FDA approved". No guarantees, but even PVC
isn't completely inert in saltwater, so I dunno if I'd worry.