Given its importance I'd be thinking seriously of all new chain even if you end up with less of it. We have 35metres (approx 115') plus nylon warp of about twice that and find it quite satisfactory especially with the new anchor.
Failing that you need to give the galvanising a good looking at. Any idea how old the chain is and/or when it was last checked ?
"Do you think God gets stoned? I think so... Look at the platypus." Robin Williams.