Yep - I agree with Doan and have a bit more to suggest.
As Doan said - the trace minerals that appear in your saltwater as a result of the salt mix you use are volitile. In effect they are 'used up' either directly by the tank inhabitants or by other chemical processes in the tank.
Recall that our aquariums are closed systems and as such the only link to the 'real world' is via a water change.
So my additional though: Naturally you will have to top off your tank periodically with new water. As you do this (even if you are using an RO
) you are adding elements at an increasing rate. They will continue to build up in the tank and there may be things which are not healthy but not detectable via your testing kits. For example: some forms of pesticides are not filtered by RO
units, if you have some of these in your water at a low enough level not to do any harm but you never change water in your tank they will continue to build each time you top off and may cause harm later.
Just an additional motivation.