ORP and water changes

[scooter86]

I just noticed that my ORP drops by almost 100 points when I do a water change. I do not disturb the sandbed, and I use Tropic Marin salt mixed with RODI water from a Spectrapure RODI. The replacement salt water is made up 1 week ahead of time, and is well aerated for that time. The ORP drops from about 420 to about 320. WHY? I have an ETSS 900 skimmer, and do not use ozone. Any input would be greatly appreciated! The water changes are twice monthly, 15% changes on a 180 gal tank.