Doing super frequent water changes.. what can go wrong?

[kien]

I did think about the cost and reasoned that the amount of salt that I would need would probably be near what larger systems, say, 280g and up would use on a regular basis anyway? Plus, if this did produce an ULNS then that would eliminate the need for additives, but it sounds like this wouldn't be the case

.. thinking out loud here.. I wonder what would happen if you did frequent enough water changes to the point where the water in the tank was (nearly) constantly equal to fresh saltwater. Like say 90% fresh saltwater all the time?