Copper is incredibly toxic to inverts, including freshwater shrimp, crays, crabs, and snails. Indeed if you glance at any of the "get rid of snails"-type medications in the aquarium section of your LFS
, you will see that most (if not all) of them use copper as the active ingredient.
The issue here is the fact that copper ions will slowly leach from the copper tubing into the water--the more acidic your water, the faster the leaching. Once there is copper in solution in your tank, it gets absorbed by the silicone sealant and then slowly re-released over time. So 5 years down the road, even if you haven't had that piece of copper in your tank in years, you might decide to add some ghost shrimp or whatever, and the next morning wake up to a bunch of dead carcasses. Copper in the aquarium is kind of like PCB's in the environment...once it is there, it takes forever-and-a-half to get rid of it.
I don't think fish are sensitive to copper in the same way that inverts are, so if you are 100% positive that you are never going to want anything other than fish in the tank, you are probably in fine shape.