I am not a salty, but thinking about your question and what I would do.
I would get one of those food safe tanks to premix the new water in. That way you can get it exactly the same as the main tank.
Not my image thank Google
Doesn't have to be like this one, but you get the idea. Get a tank that can hold the amount of water you want to change out. Premix in said tank to get the peramaters correct, them pump into main tank.