http://www.engineersedge.com/galvanic_capatability.htm
Copper/Nickel is about as bad as copper/brass. Even nickel/brass is rated as being OK for harsh environments. Nickel/silver is maybe getting a bit dodgy though, but again not as bad as copper/silver.
In any case, they're presumably electroplated - in which case the mass of nickel in the system will be tiny. At worst they're only going to strip the plating off the barbs (probably reacting with the barbs themselves actually).