You can use your main tank water, just drain the water from the main tank and replace the same amount with kH buffered fresh water. Meaning to add kH buffer till the fresh water kH is around 10. This is to prevent the pH in the low salinity water from crashing hence killing your lifestock.
You will need to remove your inverts else they will say bye bye to you.
Your beneficial bacterials will survive the hyposalinity treatment.
Do a 20% water change with freshwater each time 2 times a day for 2 days will drop your salinity to 14ppt.
All the best, keep us updated. You'll need to keep the low salinity for at least 6 weeks. Meanwhile you'll have to monitor the salinity level carefully so that it does not exceed 16ppt else you'll have to start the 6 weeks countdown all over again. If you need to top up the water that is evaporated then you need to add kH buffered freshwater with kH ard 10.
A refractometer will be best to check the salinity as they are most accurate.