Well, as Denny explained, I wouldn't use salinity as a means to control alk. Use a different salt if it's that important. RBS is low, and other than costing more, nothing wrong with it. You don't NEED alk to be low just to run zeo, running it a bit higher is fine.
But, your whole tank will be at whatever level you set the alk. Say 7. If you then do a 20% water change with IO at 8.5, the net result will be pretty darn close to 7 and will balance out quickly with the system.
But, you do seem insistent on running low salt, so given that, I don't think anything terrible will happen. Just not something I would personally do.
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Brad
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