It can be done in a day. When doing a hyposalinity treatment for Ich it can take 2 or more days to get down to 1.009. This is fine for a fish in QT.
Here are the instructions for doing that, though you want to stop at step 1.
The salinity must be lowered gradually to give the fish time to adjust to the lower salinity and more importantly ensures the bacteria in the biological filter can adjust. It should take around 2 days to get from 35‰ to 14‰.
Your starting point should be between 1.025 and 1.027. Replace about one fifth of the volume with RO
or aged freshwater that has been well aerated. Repeat this 12, 24 and 36 hours later, monitoring the specific gravity along the way. After the fourth water change the specific gravity should be 1.010 or pretty close. Wait a few hours to make the final adjustment to get down to 1.009. Note that you can estimate the resulting specific gravity. If you are changing one fifth of the water and the current specific gravity is 1.025 the result will be:
- ((1.025*4)+1.000)/5 = 1.020 approx.
Then, after 12 hours:
- ((1.020*4)+1.000)/5 = 1.016 approx.
After 24 hours:
- ((1.016*4)+1.000)/5 = 1.013 approx.
After the 4th change:
- ((1.013*4)+1.000)/5 = 1.010 approx.