The biggest advantage to using existing tank water is that the water chemistry in the QT will very closely match that of the Main tank so it will minimise the adjustment needed by the fish when they come out of the QT.
The other benefit is that the existing tank water is already laden with the biological residue that decomposes into Amonia and Nitrite so you can start growing both cultures right away.
Starting from scratch on a QT, I would use 50% main tank water and 50% new water (my QT is 30 gal so I would have to do a 50% water change to the Maint tank to fill it. This would stress my main system too much).
I feed the tank with a coctail shrimp to provide amonia. To jump start the process I have taken a small babyfood jar of sand from the tank and placed it in the QT. (Keeping the sand in the jar since the QT is bare bottomed.)
Once the tank has cycled, I check the Nitrates. I then perform 10-20% water changes using main tank water over the course of a couple of days to get the nitrates under 10ppm. While the tank is up I do a 10% water change every week using water from the main tank.
As an added bonus my main tank gets lots of fresh seawater in small doses over time.
I do not clean anything except the glass in the QT until I empty it.
I hope that helps.
Jim
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