I couldn't imagine lugging 210gals from the LFS/Wal-Mart. I had to fill my 40 gallon sump, I added to my 29gal, it took 4 trips to the LFS. I winded up spending almost an complete RO/DI cost on gas (this was way back too), extra containers, etc. In all honesty, I would just by a new RO/DI geared for aquaria use (not a general G&E or whatever). I got 600gals before I had update my filters. If you calculate that out $150/600, equals 20¢ per gal. Cheaper then Wal-Mart. Not to mention a complete filter update, 3 prefilters, RO membrane, and DI resin, is only about $60. So from here out, it literally costs me pennies for water.
I did the 'freshwater fill' on my 125 as well. Just used the RO/DI unit to fill the tank, sump, and filters (skimmer), and run the tank. No leaks, so I added salt. Then added sand and rocks.