My tank records lead me to believe the average TDS drop here (mineral) is in the 35 to 50ppm range and to date I've never had a critter problem I could attribute to osmotic shock. That drop only last a few hours to overnight at most as the minerals mix and content is restored.
The Fraternity of Dirt
If at first you don't succeed,,, keep kicking it RubberSideDownOnTheLanding, 2-75g planted, 5-55g planted, 5-20g planted, 110g w/30g sump, 8-10g, Refugium, doghouse/newbie 2012 update adding table top pleco pans & a 90g (Nutz)
I to weekly water changes on Sunday morning, 50 gallon long using R/O water. I test before and pull out 10-15 gallons. TDS goes from 750 to mid 500s but it back to 700ish within an hour or so. I've been doing it this way for about 6 months with no issues. Fish are fine. PH, ammonia, nitrite, nitrate have all been consistant after the initial cycle.
If you need to drop your TDS by a lot for whatever reason, do it slowly, over the course of several hours. Dripping new water into the tank that's got drastically different parameters (slowly dripping for a few hours) will alleviate many issues.
With the meter you will be able to spot check tanks quickly.
I check TDS more often than anything else, because with the meter it might take me 5 minutes to check the 11 tanks in the dining room. I have a small dry erase board on the rack that I keep track of the last 3 readings, which will show me if a tank is ramping up faster than normal. In those cases I break out the rest of the drop tests for closer inspection to see what is giving way.