A quick note on Zinc,
Its effects on the environment are forcing the EU parliament to consider forcing all galvanising plants to paint the work after dipping it in the zinc bath to prevent the zinc leaching into waterways.
The rust of stainless steel is not a problem. If you look at a load of mature marine systems (generally calssed as more sensitive than FW) they will have rusting jubilee clips on the return pumps etc. with absolutely no effect on the system.