I believe when Dave says "aggressive" he means that after water has been filtered and deionized, it has a MUCH greater affinity to anything that can be dissolved. Tap water won't corrode a brass valve very quickly because it's already saturated with minerals; DI water has all kinds of room to absorb whatever ions it can get ahold of, so it will corrode a metal valve or fitting much faster.
Short contact time doesn't help all that much; regardless of how fast water is flowing, the water at the inside surface of a pipe or fitting has zero velocity; this will do any necessary reacting with the surface metal and diffuse the ions into the passing water that contacts it. Considering just how sensitive our reefs can be to copper in the water I would think that rather than debating this, one would just pay a few dollars for a plastic or stainless valve on the output side and have some peace of mind.
Been awhile since my chemistry days but this much I remember, even if I didn't say it right.
[ September 13, 2001: Message edited by: dbman ]