It's kinda gettin silly.
First and formost, if you test the bucket of replacemnt water and the tank water with the same piece of equipment (float, swing arm, refractomerer, picnometer what ever..) and the slainity is close or the same great!. If you tank/livestock is doing well at .024 great that means every thing is aclimated to that salinity. If its .019 and every thing is thriving so be it. It simply must be consisitent or a slow change to allow everything to adapt.
If you are having a problem, then worst case senairo take a tank water sample to a local store that has a refracometere and have them test it. Then test with your apparatus. Bob's your uncle. make the changes you need to, and re-test your corrections (for aparatus of choice) on a regular basis.
Easy.
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|NAS- If it's not broke, don't fix it.
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