One thing to keep in mind is that the maximum rate for DOS is 1.3 gallons/hour, so depending on the size of your tank, a 10% water change may not be practical. For example, if you have a 120 gallon system, 10% = 12 gallons = 9.2 hours, and that's if you have both the DOS drain and fill running at the same time, which means you're not really doing a 10% change because some of the water you drain is actually brand new water that you just added. To do a true 10% change, you'd need to run DOS for 9.2 hours to drain the water, then run it another 9.2 hours to refill it. Technically it can be done, but think of the other issues that raises like your ATO thinking the water level is too low and trying to refill with fresh water, or the skimmer level being too low for so many hours, or will your return pump need to shut down because the sump level is too low for several hours?
I prefer to do small daily water changes, and that's what is recommended if using DOS for auto water changes. I drain 1.5 gallons over a 1.5 hour period, then I add 1.5 gallons over the next 1.5 hour period. In my Reefer 425 XL, this works out to approximately 1.3% per day or 10% per week. Because it's running for a shorter period of time and working with a smaller volume of water, it doesn't impact my skimmer or return pump, and I simply program the ATO to be disabled for 3 hours so there really isn't a big swing in salinity.
