aluminium
aluminium breaks down slightly in saltwater, even though it doesn't rust, in a small environment like a tank it becomes deadly for inverts...thats why they usually coat any alum parts in working pumps, etc. It doesn't effect vertebrates that much, so often a dealer will sell a standard chiller to an invert keeper innocently. That is why it pays to shop at a local store where they keep reef tanks, and know what they are doing...