Water Options - Initial Tank Fill

[Boulder]

Hello all - after considering various new systems for my start up FOWLER aquarium, I have settled on a 40-gallon glass tank. I haven't set anything up yet as I want to have a plan in place but my question today is about water.

I have obviously learned tap water in a no-no and where I live, city water is extremely high is phosphates so that could be a disastrous start.

So my LFS sells pre-mixed water so that is certainly a good option. But yesterday I noticed at another one of our LFS they sell boxes of pre-mixed salt water - I believe it was made by Oceanic. For the initial fill would this work and does anyone have experience with these premixed boxes?