To use old water is a good idea in many aspects. One - often overlooked reason is that old water contain many biological active substances that protects the mucus layer of the fish. It also content particles with nitrifying bacteria attached on. New mixed saltwater is - IMO - very often too chemical for the well-being of the fish. If you mix 75 % old and 25 new - it will be as you did a 25 % WC on your old aquarium. I would also transfer some sand and rocks to the new started aquarium. Start your feeding of the new inhabitants very slowly and you can ad them directly.
If you heat and have the water in motion (with a pump) - it could be there for ever.
This is the way I always have done when starting new aquariums if I have an old well function aquarium on hand,
Sincerely Lasse