The one thing I'm really not worried about with fish is salinity. Teleost fish can handle rapid changes in salinity just fine, especially in the range from 1.016 to 1.030. A change of 0.010 is not an issue within that range. I even bought fish down from normal salinity (1.026) to therapeutic hyposalinity (1.008 - 1.010) by just dropping them into the hyposaline tank. The 0.001 change per day applies only when bring fish back up after being for weeks in hyposalinity, and only until you get to 1.016.
That said, freshwater dips are harsh to fish, especially smaller ones. The parameters to match here are primarily pH and temperature. Also be sure that the water is well dechlorinated.
A freshwater dip should be at least 10 minutes to be conclusive about flukes, though that can be quite stressful even for bigger fish. Tiny or even small fish i would probably not expose to this.
In any case the fish need to be closely monitored and brought back to normal salinity when they show serious signs of distress.
But don't be fooled by some initial panic or fish playing dead at the start of the bath.
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