75 gallons. So I assumed you just wanted to have on hand for makeup water from testing, skimmate removal, cleaning sand bed and that we were talking nano sized. I'm not sure why you want to start a water change if you haven't been doing them and it's working for you.
But if you do, you'd be safe adding one container (4.4 gallons I read) at a time (it's not going to change salinity that much). Realize though that doing a 4.4 gallon WC once a week for a year you'd have about 95% (52 changes) of the water swapped out.
However, I think that makes it financially problematic (would for me).
Another small volume option - is to go with ESV Salt Mix - you can buy 50 gallons mix at a time - designed for a 5 gallon bucket as it's extremely uniform.
Here's a handy
calculator I use (Hamza's Reef) for water change and salinity adjustment.