Adding larger sump to SPS tank, have a question

[svavrek]

I have a 46g bowfront tank that I decided to make mostly SPS a while back and while everything is doing good, I want to put a larger sump under it... except I cannot fit anything more than a 10g underneath, so I want to plumb it thru the wall to the other side of the basement and use maybe a 40BR or 75g as a new sump. Should I just fill the sump with water FROM the tank over a few weeks worth of water changes and then make it active when I have enough water in it? I do not want to use all new water, of course, but don't want to put too much or any undue stress on the corals... What are your thoughts?

 

[bobssecrtsn]

Use new water, match sg and temp. Try to use a salt brand that matches your Big 3s