Wright Huntley  wrote:

>The infrequent mention of the use of PVC, here, tends to make me 
>nervous. There is a special formulation of PVC called "CPVC" that is 
>always required for "potable water" applications. IMO, that's the only 
>kind we should consider for any extended contact with our tank water.

In a previous life, I managed the operation of a PVC plant and a PVC 
compound plant.  PVC is safe for drinking water provided that you buy (IN 
the USA) pipe and fittings marked, "NSF-pw."  This is pipe that is made 
from a compound that has passed tests of the National Sanitary Foundation 
that show it is safe for drinking water and presumably for aquatic critters 
too. This is usually the white pipe.  The grey pipe is usually not 
considered suitable for drinking water and should be avoided in the 
aquarium as well. 

The pipe compounds (the compound gets extruded to make pipe or injection 
molded to make the fittings) we made contained: PVC resin, CaCO3, TiO2, 
waxes, and a tin based stabilizer.  The tin stabilizer is the most toxic 
substance and does not migrate from the pipe to the water.  One NSF test is 
to test the pipe with acidic water and analyze the water for excessive 
levels of tin.  

As a precaution, you should always rinse the pipe and fittings before use 
as there are small residues of tin chlorides which form during the 
extrusion process and waxes on the surface of the pipe/fittings. These 
rinse off easily.

CPVC is a co-polymer made from vinyl chloride and vinylidene monomers and 
is stronger than PVC and uses similar compounding agents.  

Doug Skokna in warm, humid, rainy Houston, TX