@Ola DantisLeaks are normally at the evaporators due to the condensation over the years and corrosion.

I had a plumber somewhat recently say that due to corrosion and the age of the duplex, he estimates I'd have to re-pipe both units in 5+ years, briefly estimating the job to cost $12k PER unit. 

If the water's that hard, you're going to be having routine trouble with mineral buildup and corrosion in all your plumbing that you're going to have to be on top of.