You are correct that it is just water and NOT salt leaving. Only top off your system with plain old RODI water (NO SALT IN IT).
About your PH question. Good question. I would top off using RODI water and test your system for PH to see the changes.
Just a quick follow up.
I have now been making my own RODI water including SW for my water change.
I am getting 000TDS out of my unit and did my water change last night (way after the LFS closed) with a big stupid grin on my face.
Why the heck didn't someone make me do this sooner...
Two weeks is too soon to be adding livestock to your tank.
What you are seeing on the sandbed my be a diatom bloom (normal for the tank cycle) and what is on the rock is probably some sort of algae.
I suspect you are still cycling your tank (posting param readings will probably show that)...
Just got in my new 5 Plus unit from Bulk Reef Supply (thanks astieti). Man does thing thing rock!
I can't recommend this thing more.
Everything was in the box that i needed to hook it up. Simple as follow the instructions!
I am finally making my own water. What a happy day.
Thanks...